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                              TRANSLATIONS

                                   OF

                       FOREIGN BIOLOGICAL MEMOIRS

                                  IV.


------------------------------------------------------------------------







                                 London

                             _HENRY FROWDE_

                   OXFORD UNIVERSITY PRESS WAREHOUSE

                           AMEN CORNER, E.C.

------------------------------------------------------------------------




                          ESSAYS UPON HEREDITY

                              AND KINDRED

                          BIOLOGICAL PROBLEMS

                                   BY

                          Dr. AUGUST WEISMANN

          PROFESSOR IN THE UNIVERSITY OF FREIBURG IN BREISGAU


                        _AUTHORISED TRANSLATION_

                               EDITED BY

                EDWARD B. POULTON, M.A., F.L.S., F.G.S.
                     TUTOR OF KEBLE COLLEGE, OXFORD
           LECTURER IN NATURAL SCIENCE, JESUS COLLEGE, OXFORD

                        SELMAR SCHÖNLAND, Ph.D.
   SUB-CURATOR OF THE FIELDING HERBARIUM IN THE UNIVERSITY OF OXFORD

                                  AND

                    ARTHUR E. SHIPLEY, M.A., F.L.S.
           FELLOW AND LECTURER OF CHRIST'S COLLEGE, CAMBRIDGE
   DEMONSTRATOR OF COMPARATIVE ANATOMY IN THE UNIVERSITY OF CAMBRIDGE




                                 Oxford

                         AT THE CLARENDON PRESS

                                  1889





------------------------------------------------------------------------

                           AUTHOR’S PREFACE.

The essays which now appear for the first time in the form of a single
volume were not written upon any prearranged plan, but have been
published separately at various intervals during the course of the last
seven years. Although when writing the earlier essays I was not aware
that the others would follow, the whole series is, nevertheless,
closely connected together. The questions which each essay seeks to
explain have all arisen gradually out of the subjects treated in the
first. Reflecting upon the causes which regulate the duration of life
in various forms, I was drawn on to the consideration of fresh
questions which demanded further research. These considerations and the
results of such research form the subject-matter of all the subsequent
essays.

I am here making use of the word ‘research’ in a sense somewhat
different from that in which it is generally employed in natural
science; for it is commonly supposed to imply the making of new
observations. Some of these essays, especially Nos. IV, V, and VI,
essentially depend upon new discoveries. But in most of the remaining
essays the researches are of a more abstract nature, and consist in
bringing forward new points of view, founded upon a variety of
well-known facts. I believe, however, that the history of science
proves that advance is not only due to the discovery of new facts, but
also to their correct interpretation: a true conception of natural
processes can only be arrived at in this way. It is chiefly in this
sense that the contents of these essays are to be looked upon as
research.

The fact that they contain the record of research made it impossible to
introduce any essential alterations in the translation, even in those
points about which my opinion has since changed to some extent. I
should to-day express some of the points in Essays I, IV, and V,
somewhat differently; but had I made such alterations, the relation
between the essays as a whole would have been rendered less clear, for
each of the earlier ones formed the foundation of that which succeeded
it. Even certain errors of interpretation are on this account left
uncorrected. Thus, for instance, in Essay IV it is assumed that the two
polar bodies expelled by sexual eggs are identical; for at that time
there was no reason for doubting that they were physiologically
equivalent. The discovery of the numerical law of the polar bodies
described in Essay VI, led to what I believe to be a truer knowledge of
them. In this way the causes of parthenogenesis, as developed in Essay
V, received an important addition in the fact published in Essay VI,
that only one polar body is expelled by parthenogenetic eggs. This fact
alone explains why sexual eggs cannot as a rule develope without
fertilization.

Hence the reader must not take the individual essays as the full and
complete expression of my present opinion; but they must rather be
looked upon as stages in research, as steps towards a more perfect
knowledge.

I must therefore express the hope that the essays may be read in the
same order as that in which they appeared, and in which they are
arranged in the present volume. The reader will then follow the same
road which I traversed in the development of the views here set forth;
and even though he may be now and then led away from the direct route,
perhaps such deviations may not be without interest.

I should wish to express my warm thanks to Mr. Poulton for the great
trouble he has taken in editing the translation, which in many places
presented exceptional difficulties. The greater part of the text I have
looked through in proof, and I believe that it well expresses the sense
of the original; although naturally I cannot presume to judge
concerning the niceties of the English language. I am especially
grateful to the three gentlemen who have brought these essays before an
English public, because I believe that many English naturalists, even
when thoroughly conversant with the German tongue, might possibly
misinterpret many points in the original; for the difficulty of the
questions treated of greatly increases the difficulty of the language.

If the readers of this book only feel half as much pleasure in its
perusal as I experienced in writing it, I shall be more than satisfied.

                                                    AUGUST WEISMANN.

    Freiburg I. Breisgau,
      _January, 1889_.

------------------------------------------------------------------------

                           EDITORS’ PREFACE.

The attention of English biologists and men of science was first called
to Professor Weismann’s essays by an article entitled ‘Death’ in ‘The
Nineteenth Century’ for May, 1885, by Mr. A. E. Shipley. Since then the
interest in the author’s arguments and conclusions has become very
general; having been especially increased by Professor Moseley’s two
articles in ‘Nature’ (Vol. XXXIII, p. 154, and Vol. XXXIV, p. 629), and
by the discussion upon ‘The Transmission of Acquired Characters,’
introduced by Professor Lankester at the meeting of the British
Association at Manchester in 1887,—a discussion in which Professor
Weismann himself took part. The deep interest which has everywhere been
expressed in a subject which concerns the very foundations of
evolution, has encouraged the Editors to hope that a volume containing
a collection of all Professor Weismann’s essays upon heredity and
kindred problems would supply a real want. At the present time, when
scientific periodicals contain frequent references to these essays, and
when the various issues which have been raised by them are discussed on
every occasion at which biologists come together, it is above all
things necessary to know exactly what the author himself has said. And
there are many signs that discussion has already suffered for want of
this knowledge.

A translation of Essays I and II was commenced by Mr. A. E. Shipley
during his residence at Freiburg in the winter of 1884. His work was
greatly aided by the kind assistance of Dr. van Rees of Amsterdam, to
whom we desire to express our most sincere thanks. The translation was
laid aside until the summer of 1888, when Mr. Shipley was invited to
co-operate with the other editors in the preparation of the present
volume; the Clarendon Press having consented to publish the complete
series of essays as one of their Foreign Biological Memoirs.

We think it probable that this work may interest many who are not
trained biologists, but who approach the subject from its philosophical
or social aspects. Such readers would do well to first study Essays I,
II, VII, and VIII, inasmuch as some preparation for the more technical
treatment pursued in the other essays will thus be gained.

The notes signed A. W. and dated, were added by the author during the
progress of the translation. The notes included in square brackets were
added by the Editors; the authorship being indicated by initials in all
cases.

In conclusion, it is our pleasant duty to thank those who have kindly
helped us by reading the proof-sheets and making valuable suggestions.
Our warmest thanks are due to Mrs. Arthur Lyttelton, Mr. W. Hatchett
Jackson, Deputy Linacre Professor in the University of Oxford, Mr. J.
S. Haldane, and Professor R. Meldola. Important suggestions were also
made by Professor E. Ray Lankester, Mr. Francis Galton, and Dr. A. R.
Wallace. Professor W. N. Parker also greatly helped us by looking over
the proof-sheets with Professor Weismann.

                                                          E. B. P.
                                                          S. S.
                                                          A. E. S.

  Oxford, _February, 1889_.






------------------------------------------------------------------------

                               CONTENTS.

   _Translator._ A. E. Shipley

               I. The Duration of Life, 1881                   1
              II. On Heredity, 1883                           67
             III. Life and Death, 1883                       107

   _Translator._ Selmar Schönland

              IV. The Continuity of the Germ-plasm as the    161
                  Foundation of a Theory of Heredity, 1885

               V. The Significance of Sexual Reproduction    251
                  in the Theory of Natural Selection, 1886

              VI. On the Number of Polar Bodies and their    333
                  Significance in Heredity, 1887

             VII. On the Supposed Botanical Proofs of the    385
                  Transmission of Acquired Characters,
                  1888.

            VIII. The Supposed Transmission of               419
                  Mutilations, 1888






------------------------------------------------------------------------






   _Abstracts of Professor Weismann’s Essays on Heredity and Kindred
             Problems, already Published in this Country._


           I. A short abstract in ‘Nature,’ Vol. XXXVII, pp.
              541-542, by P. C. Mitchell.

          II. A short abstract in ‘Nature,’ Vol. XXXVIII, pp.
              156-157, by P. C. Mitchell.

         III. A short article on the subject of this Essay in
              ‘The Nineteenth Century’ for May, 1885, by A. E.
              Shipley.

          IV. Abstract in ‘Nature,’ Vol. XXXIII, pp. 154-157, by
              Professor Moseley.

           V. Abstract in ‘Nature,’ Vol. XXXIV, pp. 629-632, by
              Professor Moseley.

          VI. Abstract in ‘Nature,’ Vol. XXXVI, pp. 607-609, by
              Professor Weismann.

         VII, VIII.  The Essays being of so recent a date no
              abstract has yet appeared in this country.

A criticism of Professor Weismann’s theories will be found in ‘The
Physiology of Plants,’ by Professor Vines, Lecture XXIII, pp. 660 et
seqq.






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                                   I.

                         THE DURATION OF LIFE.

                                 1881.




------------------------------------------------------------------------

                         THE DURATION OF LIFE.

                                PREFACE.

The following paper was read at the meeting of the Association of
German Naturalists at Salzburg, on September 21st, 1881; and it is here
printed in essentially the same form. A somewhat longer discussion of a
few points has been now intercalated; these were necessarily omitted
from the lecture itself for the sake of brevity, and are, therefore,
not contained in the account printed in the Proceedings of the
fifty-fourth meeting of the Association.

Further additions would not have been admissible without an essential
change of form, and therefore I have not put into the text a note which
ought otherwise to have been there, and which is now to be found in the
Appendix, as Note 8. It fills up a gap which was left in the text, for
the above-mentioned reason, by attempting to give an explanation of the
normal death of cells of tissues—an explanation which is required if we
are to maintain that unicellular organisms are so constituted as to be
potentially immortal.

The other parts of the Appendix contain, partly further expansions,
partly proofs of the views brought forward in the text, and above all a
compilation of all the observations which are known to me upon the
duration of life in several groups of animals. I am indebted to several
eminent specialists for the communication of many data, which are among
the most exact that I have been able to obtain. Thus Dr. Hagen of
Cambridge (U.S.A.) was kind enough to send me an account of his
observations upon insects of different orders: Mr. W. H. Edwards of
West Virginia, and Dr. Speyer of Rhoden—their experience with
butterflies. Dr. Adler of Schleswig sent me data upon the duration of
life in _Cynipidae_, which have a special value, as they are
accompanied by very exact observations upon the conditions of life in
these animals; hence in this case we can directly examine the factors
upon which, as I believe, the duration of life is chiefly based. Sir
John Lubbock in England, and Dr. August Forel of Zürich, have had the
kindness to send me an account of their observations upon ants, and S.
Clessin of Ochsenfurth his researches upon our native land and
fresh-water Mollusca.

In publishing these valuable communications, together with all facts
which I have been able to collect from literature upon the subject of
the duration of life, and the little which I have myself observed upon
this subject, I hope to provide a stimulus for further observation in
this field, which has been hitherto much neglected. The views which I
have brought forward in this paper are based on a comparatively small
number of facts, at least as far as the duration of life in various
species is concerned. The larger the number of accurate data which are
supplied, and the more exactly the duration of life and its conditions
are ascertained, the more securely will it be possible to establish our
views upon the causes which determine the duration of life.

                                                             A. W.

      Naples, _Dec. 6, 1881_.

                                   I.

                         THE DURATION OF LIFE.

With your permission, I will bring before you to-day some thoughts upon
the subject of the duration of life. I can scarcely do better than
begin with the simple but significant words of Johannes Müller:
‘Organic bodies are perishable; while life maintains the appearance of
immortality in the constant succession of similar individuals, the
individuals themselves pass away.’

Omitting, for the time being, any discussion as to the precise accuracy
of this statement, it is at any rate obvious that the life of an
individual has its natural limit, at least among those animals and
plants which are met with in every-day life. But it is equally obvious
that the limits are very differently placed in the various species of
animals and plants. These differences are so manifest that they have
given rise to popular sayings. Thus Jacob Grimm mentions an old German
saying, ‘A wren lives three years, a dog three times as long as a wren,
a horse three times as long as a dog, and a man three times as long as
a horse, that is eighty-one years. A donkey attains three times the age
of a man, a wild goose three times that of a donkey, a crow three times
that of a wild goose, a deer three times that of a crow, and an oak
three times the age of a deer.’

If this be true a deer would live 6000 years, and an oak nearly 20,000
years. The saying is certainly not founded upon exact observation, but
it becomes true if looked upon as a general statement that the duration
of life is very different in different organisms.

The question now arises as to the causes of these great differences.
How is it that individuals are endowed with the power of living long in
such very various degrees?

One is at first tempted to seek the answer by an appeal to the
differences in morphological and chemical structure which separate
species from one another. In fact all attempts to throw light upon the
subject which have been made up to the present time lie in this
direction.

All these explanations are nevertheless insufficient. In a certain
sense it is true that the causes of the duration of life must be
contained in the organism itself, and cannot be found in any of its
external conditions or circumstances. But structure and chemical
composition—in short the physiological constitution of the body in the
ordinary sense of the words—are not the only factors which determine
duration of life. This conclusion forces itself upon our attention as
soon as the attempt is made to explain existing facts by these factors
alone: there must be some other additional cause contained in the
organism as an unknown and invisible part of its constitution, a cause
which determines the duration of life.

The size of the organism must in the first place be taken into
consideration. Of all organisms in the world, large trees have the
longest lives. The Adansonias of the Cape Verd Islands are said to live
for 6000 years. The largest animals also attain the greatest age. Thus
there is no doubt that whales live for some hundreds of years.
Elephants live 200 years, and it would not be difficult to construct a
descending series of animals in which the duration of life diminishes
in almost exact proportion to the decrease in the size of the body.
Thus a horse lives forty years, a blackbird eighteen, a mouse six, and
many insects only a few days or weeks.

If however the facts are examined a little more closely it will be
observed that the great age (200 years) reached by an elephant is also
attained by many smaller animals, such as the pike and carp. The horse
lives forty years, but so does a cat or a toad; and a sea anemone has
been known to live for over fifty years. The duration of life in a pig
(about twenty years) is the same as that in a crayfish, although the
latter does not nearly attain the hundredth part of the weight of a pig.

It is therefore evident that length of life cannot be determined by the
size of the body alone. There is, however, some relation between these
two attributes. A large animal lives longer than a small one because it
is larger; it would not be able to become even comparatively large
unless endowed with a comparatively long duration of life.

Apart from all other reasons, no one could imagine that the gigantic
body of an elephant could be built up like that of a mouse in three
weeks, or in a single day like that of the larva of certain flies. The
gestation of an elephant lasts for nearly two years, and maturity is
only reached after a lapse of about twenty-four years.

Furthermore, to ensure the preservation of the species, a longer time
is required by a large animal than by a small one, when both have
reached maturity. Thus Leuckart and later Herbert Spencer have pointed
out that the absorbing surface of an animal only increases as the
square of its length, while its size increases as the cube; and it
therefore follows that the larger an animal becomes, the greater will
be the difficulty experienced in assimilating any nourishment over and
above that which it requires for its own needs, and therefore the more
slowly will it reproduce itself.

But although it may be stated generally that the duration of the period
of growth and length of life are longest in the largest animals, it is
nevertheless impossible to maintain that there is any fixed relation
between the two; and Flourens was mistaken when he considered that the
length of life was always equivalent to five times the duration of the
period of growth. Such a conclusion might be accepted in the case of
man if we set his period of growth at twenty years and his length of
life at a hundred; but it cannot be accepted for the majority of other
Mammalia. Thus the horse lives from forty to fifty years, and the
latter age is at least as frequently reached among horses as a hundred
years among men; but the horse becomes mature in four years, and the
length of its life is thus ten or twelve times as long as its period of
growth.

The second factor which influences the duration of life is purely
physiological: it is the rate at which the animal lives, the rapidity
with which assimilation and the other vital processes take place. Upon
this point Lotze remarks in his Microcosmus—‘Active and restless
mobility destroys the organized body: the swift-footed animals hunted
by man, as also dogs, and even apes, are inferior in length of life to
man and the larger beasts of prey, which satisfy their needs by a few
vigorous efforts.’ ‘The inertness of the Amphibia is, on the other
hand, accompanied by relatively great length of life.’

There is certainly some truth in these observations, and yet it would
be a great mistake to assume that activity necessarily implies a short
life. The most active birds have very long lives, as will be shown
later on: they live as long as and sometimes longer than the majority
of Amphibia which reach the same size. The organism must not be looked
upon as a heap of combustible material, which is completely reduced to
ashes in a certain time the length of which is determined by size, and
by the rate at which it burns; but it should be rather compared to a
fire, to which fresh fuel can be continually added, and which, whether
it burns quickly or slowly, can be kept burning as long as necessity
demands.

The connection between activity and shortness of life cannot be
explained by supposing that a more rapid consumption of the body
occurs, but it is explicable because the increased rate at which the
vital processes take place permit the more rapid achievement of the aim
and purpose of life, viz. the attainment of maturity and the
reproduction of the species.

When I speak of the aim and purpose of life, I am only using figures of
speech, and I do not mean to imply that nature is in any way working
consciously.

When I was speaking of the relation between duration of life and the
size of the body, I might have added another factor which also exerts
some influence, viz. the complexity of the structure. Two organisms of
the same size, but belonging to different grades of organization, will
require different periods of time for their development. Certain
animals of a very lowly organization, such as the Rhizopoda, may attain
a diameter of ·5 mm. and may thus become larger than many insects’
eggs. Yet under favourable circumstances an Amoeba can divide into two
animals in ten minutes, while no insect’s egg can develope into the
young animal in a less period than twenty-four hours. Time is required
for the development of the immense number of cells which must in the
latter case arise from the single egg-cell.

Hence we may say that the peculiar constitution of an animal does in
part determine the length of time which must elapse before reproduction
begins. The period before reproduction is however only part of the
whole life of an animal, which of course extends over the total period
during which the animal exists.

Hitherto it has always been assumed that the duration of this total
period is solely determined by the constitution of the animal’s body.
But the assumption is erroneous. The strength of the spring which
drives the wheel of life does not solely depend upon the size of the
wheel itself or upon the material of which it is made; and, leaving the
metaphor, duration of life is not exclusively determined by the size of
the animal, the complexity of its structure, and the rate of its
metabolism. The facts are plainly and clearly opposed to such a
supposition.

How, for instance, can we explain from this point of view the fact that
the queen-ant and the workers live for many years, while the males live
for a few weeks at most? The sexes are not distinguished by any great
difference in size or complexity of body, or in the rate of metabolism.
In all these three particulars they must be looked upon as precisely
the same, and yet there is this immense difference between the lengths
of their lives.

I shall return later on to this and other similar cases, and for the
present I assume it to be proved that physiological considerations
alone cannot determine the duration of life. It is not these which
alone determine the strength of the spring which moves the machinery of
life; we know that springs of different strengths may be fixed in
machines of the same kind and quality. This metaphor is however
imperfect, because we cannot imagine the existence of any special force
in an organism which determines the duration of its life; but it is
nevertheless useful because it emphasises the fact that the duration of
life is forced upon the organism by causes outside itself, just as the
spring is fixed in its place by forces outside the machine, and not
only fixed in its place, but chosen of a certain strength so that it
will run down after a certain time.

To put it briefly, I consider that duration of life is really dependent
upon adaptation to external conditions, that its length, whether longer
or shorter, is governed by the needs of the species, and that it is
determined by precisely the same mechanical process of regulation as
that by which the structure and functions of an organism are adapted to
its environment.

Assuming for the moment that these conclusions are valid, let us ask
how the duration of life of any given species can have been determined
by their means. In the first place, in regulating duration of life, the
advantage to the species, and not to the individual, is alone of any
importance. This must be obvious to any one who has once thoroughly
thought out the process of natural selection. It is of no importance to
the species whether the individual lives longer or shorter, but it is
of importance that the individual should be enabled to do its work
towards the maintenance of the species. This work is reproduction, or
the formation of a sufficient number of new individuals to compensate
the species for those which die. As soon as the individual has
performed its share in this work of compensation, it ceases to be of
any value to the species, it has fulfilled its duty and may die. But
the individual may be of advantage to the species for a longer period
if it not only produces offspring, but tends them for a longer or
shorter time, either by protecting, feeding, or instructing them. This
last duty is not only undertaken by man, but also by animals, although
to a smaller extent; for instance, birds teach their young to fly, and
so on.

We should therefore expect to find that, as a rule, life does not
greatly outlast the period of reproduction except in those species
which tend their young; and as a matter of fact we find that this is
the case.

All mammals and birds outlive the period of reproduction, but this
never occurs among insects except in those species which tend their
young. Furthermore, the life of all the lower animals ceases also with
the end of the reproductive period, as far as we can judge.

Duration of life is not however determined in this way, but only the
point at which its termination occurs relatively to the cessation of
reproduction. The duration itself depends first upon the length of time
which is required for the animal to reach maturity—that is, the
duration of its youth, and, secondly, upon the length of the period of
fertility—that is the time which is necessary for the individual to
produce a sufficient number of descendants to ensure the perpetuation
of the species. It is precisely this latter point which is determined
by external conditions.

There is no species of animal which is not exposed to destruction
through various accidental agencies—by hunger or cold, by drought or
flood, by epidemics, or by enemies, whether beasts of prey or
parasites. We also know that these causes of death are only apparently
accidental, or at least that they can only be called accidental as far
as a single individual is concerned. As a matter of fact a far greater
number of individuals perish through the operation of these agencies
than by natural death. There are thousands of species of which the
existence depends upon the destruction of other species; as, for
example, the various kinds of fish which feed on the countless minute
Crustacea inhabiting our lakes.

It is easy to see that an individual is, _ceteris paribus_, more
exposed to accidental death when the natural term of its life becomes
longer; and therefore the longer the time required by an individual for
the production of a sufficient number of descendants to ensure the
existence of the species, the greater will be the number of individuals
which perish accidentally before they have fulfilled this important
duty. Hence it follows, first, that the number of descendants produced
by any individual must be greater as the duration of its reproductive
period becomes longer; and, secondly, the surprising result that nature
does not tend to secure the longest possible life to the adult
individual, but, on the contrary, tends to shorten the period of
reproductive activity as far as possible, and with this the duration of
life; but these conclusions only refer to the animal and not to the
vegetable world.

All this sounds very paradoxical, but the facts show that it is true.
At first sight numerous instances of remarkably long life seem to
refute the argument, but the contradictions are only apparent and
disappear on closer investigation.

Birds as a rule live to a surprisingly great age. Even the smallest of
our native singing birds lives for ten years, while the nightingale and
blackbird live from twelve to eighteen years. A pair of eider ducks
were observed to make their nest in the same place for twenty years,
and it is believed that these birds sometimes reach the age of nearly
one hundred years. A cuckoo, which was recognised by a peculiar note in
its call, was heard in the same forest for thirty-two consecutive
years. Birds of prey, and birds which live in marshy districts, become
much older, for they outlive more than one generation of men.

Schinz mentions a bearded vulture which was seen sitting on a rock upon
a glacier near Grindelwald, and the oldest men in Grindelwald had, when
boys, seen the same bird sitting on the same rock. A white-headed
vulture in the Schönbrunn Zoological Gardens had been in captivity for
118 years, and many examples are known of eagles and falcons reaching
an age of over 100 years. Finally, we must not forget Humboldt’s[1]
Atur parrot from the Orinoco, concerning which the Indians said that it
could not be understood because it spoke the language of an extinct
tribe.

It is therefore necessary to ask how far we can show that such long
lives are really the shortest which are possible under the
circumstances.

Two factors must here be taken into consideration; first, that the
young of birds are greatly exposed to destructive agencies; and,
secondly, that the structure of a bird is adapted for flight and
therefore excludes the possibility of any great degree of fertility.

Many birds, like the stormy petrel, the diver, guillemot, and other
sea-birds, lay only a single egg, and breed (as is usually the case
with birds) only once a year. Others, such as birds of prey, pigeons,
and humming-birds, lay two eggs, and it is only those which fly badly,
such as jungle fowls and pheasants, which produce a number of eggs
(about twenty), and the young of these very species are especially
exposed to those dangers which more or less affect the offspring of all
birds. Even the eggs of our most powerful native bird of prey, the
golden eagle, which all animals fear, and of which the eyrie, perched
on a rocky height, is beyond the reach of any enemies, are very
frequently destroyed by late frosts or snow in spring, and, at the end
of the year in winter, the young birds encounter the fiercest of foes,
viz. hunger. In the majority of birds, the egg, as soon as it is laid,
becomes exposed to the attacks of enemies; martens and weasels, cats
and owls, buzzards and crows are all on the look out for it. At a later
period the same enemies destroy numbers of the helpless young, and in
winter many succumb in the struggle against cold and hunger, or to the
numerous dangers which attend migration over land and sea, dangers
which decimate the young birds.

It is impossible directly to ascertain the exact number which are thus
destroyed; but we can arrive at an estimate by an indirect method. If
we agree with Darwin and Wallace in believing that in most species a
certain degree of constancy is maintained in the number of individuals
of successive generations, and that therefore the number of individuals
within the same area remains tolerably uniform for a certain period of
time; it follows that, if we know the fertility and the average
duration of life of a species, we can calculate the number of those
which perish before reaching maturity. Unfortunately the average length
of life is hardly known with certainty in the case of any species of
bird. Let us however assume, for the sake of argument, that the
individuals of a certain species live for ten years, and that they lay
twenty eggs in each year; then of the 200 eggs which are laid during
the ten years, which constitute the lifetime of an individual, 198 must
be destroyed, and only two will reach maturity, if the number of
individuals in the species is to remain constant. Or to take a concrete
example; let us fix the duration of life in the golden eagle at 60
years, and its period of immaturity (of which the length is not exactly
known) at ten years, and let us assume that it lays two eggs a
year;—then a pair will produce 100 eggs in 50 years, and of these only
two will develope into adult birds; and thus on an average a pair of
eagles will only succeed in bringing a pair of young to maturity once
in fifty years. And so far from being an exaggeration, this calculation
rather under-estimates the proportion of mortality among the young; it
is sufficient however to enforce the fact that the number of young
destroyed must reach in birds a very high figure as compared with the
number of those which survive [See Note 1].

If this argument holds, and at the same time the fertility from
physical and other grounds cannot be increased, it follows that a
relatively long life is the only means by which the maintenance of the
species of birds can be secured. Hence a great length of life is proved
to be an absolute necessity for birds.

I have already mentioned that these animals demonstrate most clearly
that physiological considerations do not by any means suffice to
explain the duration of life. Although all vital processes take place
with greater rapidity and the temperature of the blood is higher in
birds than in mammals, yet the former greatly surpass the latter in
length of life. Only in the largest Mammalia,—the whales and the
elephants—is the duration of life equal to or perhaps greater than that
of the longest lived birds. If we compare the relative weights of these
animals, the Mammalia are everywhere at a disadvantage. Even such large
animals as the horse and bear only attain an age of fifty years at the
outside; the lion lives about thirty-five years, the wild boar
twenty-five, the sheep fifteen, the fox fourteen, the hare ten, the
squirrel and the mouse six years [See Note 2]; but the golden eagle,
though it does not weigh more than from 9-12 pounds, and is thus
intermediate as regards weight between the hare and the fox, attains
nevertheless an age which is ten times as long. The explanation of this
difference is to be found first in the much greater fertility of the
smaller Mammalia, such as the rabbit or mouse, and secondly in the much
lower mortality among the young of the larger Mammalia. The minimum
duration of life necessary for the maintenance of the species is
therefore much lower than it is among birds. Even here, however, we are
not yet in possession of exact statistics indicating the number of
young destroyed; but it is obvious that Mammalia possess over birds a
great advantage in their intra-uterine development. In Mammalia the
destruction of young only begins after birth, while in birds it begins
during the development of the embryo. This distinction is in fact
carried even further, for many mammals protect their young against
enemies for a long time after birth.

It is unnecessary to go further into the details of these cases, or to
consider whether and to what extent every class of the animal kingdom
conforms to these principles. Thus to consider all or even most of the
classes of the animal kingdom would be quite impossible at the present
time, because our knowledge of the duration of life among animals is
very incomplete. Biological problems have for a long time excited less
interest than morphological ones. There is nothing or almost nothing to
be found in existing zoological text books upon the duration of life in
animals; and even monographs upon single classes, such as the Amphibia,
reptiles, or even birds, contain very little on this subject. When we
come to the lower animals, knowledge on this point is almost entirely
wanting. I have not been able to find a single reference to the age in
Echinodermata, and very little about that of worms, Crustacea, and
Coelenterata [See Note 4]. The length of life in many molluscan species
is very well known, because the age can be determined by markings on
the shell [See Note 5]. But even in this group, any exact knowledge,
such as would be available for our purpose, is still wanting concerning
such necessary points as the degree of fertility, the relation to other
animals, and many other factors.

Data the most exact in all respects are found among the insects [See
Note 3], and to this class I will for a short time direct your special
attention. We will first consider the duration of larval life. This
varies very greatly, and chiefly depends upon the nature of the food,
and the ease or difficulty with which it can be procured. The larvae of
bees reach the pupal stage in five to six days; but it is well known
that they are fed with substances of high nutritive value (honey and
pollen), and that they require no great effort to obtain the food,
which lies heaped up around them. The larval life in many
_Ichneumonidae_ is but little longer, being passed in a parasitic
condition within other insects; abundance of accessible food is thus
supplied by the tissues and juices of the host. Again, the larvae of
the blow-fly become pupae in eight to ten days, although they move
actively in boring their way under the skin and into the tissues of the
dead animals upon which they live. The life of the leaf-eating
caterpillars of butterflies and moths lasts for six weeks or longer,
corresponding to the lower nutritive value of their food and the
greater expenditure of muscular energy in obtaining it. Those
caterpillars which live upon wood, such as _Cossus ligniperda_, have a
larval life of two to three years, and the same is true of
hymenopterous insects with similar habits, such as _Sirex_.

Furthermore, predaceous larvae require a long period for attaining
their full size, for they can only obtain their prey at rare intervals
and by the expenditure of considerable energy. Thus among the
dragon-flies larval life lasts for a year, and among many may-flies
even two or three years.

All these results can be easily understood from well-known
physiological principles, and they indicate that the length of larval
life is very elastic, and can be extended as circumstances demand; for
otherwise carnivorous and wood-eating larvae could not have survived in
the phyletic development of insects. Now it would be a great mistake to
suppose that there is any reciprocal relation between duration of life
in the larva and in the mature insect, or imago; or, to put it
differently, to suppose that the total duration of life is the same in
insects of the same size and activity, so that the time which is spent
in the larval state is, as it were, deducted from the life of the
imago, and _vice versa_. That this cannot be the case is shown by the
fact already alluded to, that among bees and ants larval life is of the
same length in males and females, while there is a difference of some
years between the lengths of their lives as imagos.

The life of the imago is generally very short, and not only ends with
the close of the period of reproduction, as was mentioned above, but
this latter period is also itself extremely short [See Note 3].

The larva of the cockchafer devours the roots of plants for a period of
four years, but the mature insect with its more complex structure
endures for a comparatively short time; for the beetle itself dies in
about a month after completing its metamorphosis. And this is by no
means an extreme case. Most butterflies have an even shorter life, and
among the moths there are many species (as in the _Psychidae_) which
only live for a few days, while others again, which reproduce by the
parthenogenetic method, only live for twenty-four hours. The shortest
life is found in the imagos of certain may-flies, which only live four
to five hours. They emerge from the pupa-case towards the evening, and
as soon as their wings have hardened, they begin to fly, and pair with
one another. Then they hover over the water; their eggs are extruded
all at once, and death follows almost immediately.

The short life of the imago in insects is easily explained by the
principles set forth above. Insects belong to the number of those
animals which, even in their mature state, are very liable to be
destroyed by others which are dependent upon them for food; but they
are at the same time among the most fertile of animals, and often
produce an astonishing number of eggs in a very short time. And no
better arrangement for the maintenance of the species under such
circumstances can be imagined than that supplied by diminishing the
duration of life, and simultaneously increasing the rapidity of
reproduction.

This general tendency is developed to very different degrees according
to conditions peculiar to each species. The shortening of the period of
reproduction, and the duration of life to the greatest extent which is
possible, depends upon a number of co-operating circumstances, which it
is impossible to enumerate completely. Even the manner in which the
eggs are laid may have an important effect. If the larva of the may-fly
lived upon some rare and widely distributed food-plant instead of at
the bottom of streams, the imagos would be compelled to live longer,
for they would be obliged—like many moths and butterflies—to lay their
eggs singly or in small clusters, over a large area. This would require
both time and strength, and they could not retain the rudimentary mouth
which they now possess, for they would have to feed in order to acquire
sufficient strength for long flights; and—whether they were carnivorous
like dragon-flies, or honey-eating like butterflies—their feeding would
itself cause a further expenditure of both time and strength, which
would necessitate a still further increase in the duration of life. And
as a matter of fact we find that dragon-flies and swift-flying
hawk-moths often live for six or eight weeks and sometimes longer.

We must also remember that in many species the eggs are not mature
immediately after the close of the pupal stage, but that they only
gradually ripen during the life of the imago, and frequently, as in
many beetles and butterflies, do not ripen simultaneously, but only a
certain number at a time. This depends, first, upon the amount of
reserve nutriment accumulated in the body of the insect during larval
life; secondly, upon various but entirely different circumstances, such
as the power of flight. Insects which fly swiftly and are continually
on the wing, like hawk-moths and dragon-flies, cannot be burdened with
a very large number of ripe eggs. In these cases the gradual ripening
of the eggs becomes necessary, and involves an increase in the duration
of life. In Lepidoptera, we see how the power of flight diminishes step
by step as soon as other circumstances permit, and simultaneously how
the eggs ripen more and more rapidly, while the length of life becomes
shorter, until a minimum is reached. Only two stages in the process of
transformation can be mentioned here.

The strongest flyers—the hawk-moths and butterflies—must be looked upon
as the most specialised and highest types among the Lepidoptera. Not
only do they possess organs for flight in their most perfect form, but
also organs for feeding—the characteristic spiral proboscis or ‘tongue.’

There are certain moths (among the Bombyces) of which the males fly as
well as the hawk-moths, while the females are unable to use their large
wings for flight, because the body is too heavily weighted by a mass of
eggs, all of which reach maturity at the same time. Such species, as
for instance _Aglia tau_, are unable to distribute their eggs over a
wide area, but are obliged to lay them all in a single spot. They can
however do this without harm to the species, because their caterpillars
live upon forest trees, which provide abundant food for a larger number
of larvae than can be produced by the eggs of a single female. The eggs
of _Aglia tau_ are deposited directly after pairing, and shortly
afterwards the insect dies at the foot of the tree among the
moss-covered roots of which it has passed the winter in the pupal
state. The female moth seldom lives for more than three or four days;
but the males which fly swiftly in the forests, seeking for the less
abundant females, live for a much longer period, certainly from eight
to fourteen days[2].

The females of the _Psychidae_ also deposit all their eggs in one
place. The grasses and lichens upon which their caterpillars live grow
close at hand upon the surface of the earth and stones, and hence the
female moth does not leave the ground, and generally does not even quit
the pupa-case, within which it lays its eggs; as soon as this duty is
finished, it dies. In relation to these habits the wings and mouth of
the female are rudimentary, while the male possesses perfectly
developed wings.

The causes which have regulated the length of life in these cases are
obvious enough, yet still more striking illustrations are to be found
among insects which live in colonies.

The duration of life varies with the sex in bees, wasps, ants, and
termites: the females have a long life, the males a short one; and
there can be no doubt that the explanation of this fact is to be found
in adaptation to external conditions of life.

The queen-bee—the only perfect female in the hive—lives two to three
years, and often as long as five years, while the male bees or drones
only live four to five months. Sir John Lubbock has succeeded in
keeping female and working ants alive for seven years—a great age for
insects[3],—while the males only lived a few weeks.

These last examples become readily intelligible when we remember that
the males neither collect food nor help in building the hive. Their
value to the colony ceases with the nuptial flight, and from the point
of view of utility it is easy to understand why their lives should be
so short [See Note 7 and Note 9]. But the case is very different with
the female. The longest period of reproduction possible, when
accompanied by very great fertility, is, as a rule, advantageous for
the maintenance of the species. It cannot however be attained in most
insects, for the capability of living long would be injurious if all
individuals fell a prey to their enemies before they had completed the
full period of life. Here it is otherwise: when the queen-bee returns
from her nuptial flight, she remains within the hive until her death,
and never leaves it. There she is almost completely secure from enemies
and from dangers of all kinds; thousands of workers armed with stings
protect, feed, and warm her; and in short there is every chance of her
living through the full period of a life of normal length. And the case
is entirely similar with the female ant. In neither of these insects is
there any reason why the advantages which follow from a lengthened
period of reproductive activity should be abandoned [See Note 6].

That an increase in the length of life has actually taken place in such
cases seems to be indicated by the fact that both sexes of the
saw-flies—the probable ancestors of bees and ants—have but a short
life. On the other hand, the may-flies afford an undoubted instance of
the shortening of life. Only in certain species is life as short as I
have indicated above; in the majority it lasts for one or more days.
The extreme cases, with a life of only a few hours, form the end of a
line of development tending in the direction of a shortened life. This
is made clear by the fact that one of these may-flies (_Palingenia_)
does not even leave its pupa-skin, but reproduces in the so-called
sub-imago stage.

It is therefore obvious that the duration of life is extremely
variable, and not only depends upon physiological considerations, but
also upon the external conditions of life. With every change in the
structure of a species, and with the acquisition of new habits, the
length of its life may, and in most cases must, be altered.

In answering the question as to the means by which the lengthening or
shortening of life is brought about, our first appeal must be to the
process of natural selection. Duration of life, like every other
characteristic of an organism, is subject to individual fluctuations.
From our experience with the human species we know that long life is
hereditary. As soon as the long-lived individuals in a species obtain
some advantage in the struggle for existence, they will gradually
become dominant, and those with the shortest lives will be exterminated.

So far everything is quite simple; but hitherto we have only considered
the external mechanism, and we must now further inquire as to the
concomitant internal means by which such processes are rendered
possible.

This brings us face to face with one of the most difficult problems in
the whole range of physiology,—the question of the origin of death. As
soon as we thoroughly understand the circumstances upon which normal
death depends in general, we shall be able to make a further inquiry as
to the circumstances which influence its earlier or later appearance,
as well as to any functional changes in the organism which may produce
such a result.

The changes in the organism which result in normal death,—senility
so-called,—have been most accurately studied among men. We know that
with advancing age certain alterations take place in the tissues, by
which their functional activity is diminished; that these changes
gradually increase, and finally either lead to direct or so-called
normal death, or produce indirect death by rendering the organism
incapable of resisting injuries due to external influences. These
senile changes have been so well described from the time of Burdach and
Bichat to that of Kussmaul, and are so well known, that I need not
enter into further details here.

In answer to an inquiry as to the causes which induce these changes in
the tissues, I can only suggest that the cells which form the vital
constituents of tissues are worn out by prolonged use and activity. It
is conceivable that the cells might be thus worn out in two ways;
either the cells of a tissue remain the same throughout life, or else
they are being continually replaced by younger generations of cells,
which are themselves cast off in their turn.

In the present state of our knowledge the former alternative can hardly
be maintained. Millions of blood corpuscles are continually dying and
being replaced by new ones. On both the internal and external surfaces
of the body countless epithelial cells are being incessantly removed,
while new ones arise in their place; the activity of many and probably
of all glands is accompanied by a change in their cells, for their
secretions consist partly of detached and partly of dissolved cells; it
is stated that even the cells of bone, connective tissue, and muscle
undergo the same changes, and nervous tissue alone remains, in which it
is doubtful whether such a renewal of cells takes place. And yet as
regards even this tissue, certain facts are known which indicate a
normal, though probably a slow renewal of the histological elements. I
believe that one might reasonably defend the statement,—in fact, it has
already found advocates,—that the vital processes of the higher (i.e.
multicellular) animals are accompanied by a renewal of the
morphological elements in most tissues.

This statement leads us to seek the origin of death, not in the waste
of single cells, but in the limitation of their powers of reproduction.
Death takes place because a worn-out tissue cannot for ever renew
itself, and because a capacity for increase by means of cell-division
is not everlasting, but finite [See Note 8]. This does not however
imply that the immediate cause of death lies in the imperfect renewal
of cells, for death would in all cases occur long before the
reproductive power of the cells had been completely exhausted.
Functional disturbances will appear as soon as the rate at which the
worn-out cells are renewed becomes slow and insufficient.

But it must not be forgotten that death is not always preceded by
senility, or a period of old age. For instance, in many of the lower
animals death immediately follows the most important deed of the
organism, viz. reproduction. Many Lepidoptera, all may-flies, and many
other insects die of exhaustion immediately after depositing their
eggs. Men have been known to die from the shock of a strong passion.
Sulla is said to have died as the result of rage, whilst Leo X
succumbed to an excess of joy. Here the psychical shock caused too
intense an excitement of the nervous system. In the same manner the
exercise of intense effort may also produce a similarly fatal
excitement in the above-mentioned insects. At any rate it is certain
that when, for some reason, this effort is not made, the insect lives
for a somewhat longer period.

It is clear that in such animals as insects we can only speak
figuratively of normal death, if we mean by this an end which is not
due to accident. In these animals an accidental end is the rule, and is
therefore, strictly speaking, normal [See Note 9].

Assuming the truth of the above-mentioned hypothesis as to the causes
of normal death, it follows that the number of cell-generations which
can proceed from the egg-cell is fixed for every species, at least
within certain limits; and this number of cell-generations, if
attained, corresponds to the maximum duration of life in the
individuals of the species concerned. Shortening of life in any species
must depend upon a decrease in the number of successive
cell-generations, while conversely, the lengthening of life depends
upon an increase in the number of cell-generations over those which
were previously possible.

Such changes actually take place in plants. When an annual plant
becomes perennial, the change—one in every way possible—can only happen
by the production of new shoots, i. e. by an increase in the number of
cell-generations. The process is not so obvious in animals, because in
them the formation of young cells does not lead to the production of
new and visible parts, for the new material is merely deposited in the
place of that which is worn out and disappears. Among plants, on the
other hand, the old material persists, its cells become lignified, and
it is built over by new cells which assume the functions of life.

It is certainly true that the question as to the necessity of death in
general does not seem much clearer from this point of view than from
the purely physiological one. This is because we do not know why a cell
must divide 10,000 or 100,000 times and then suddenly stop. It must be
admitted that we can see no reason why the power of cell-multiplication
should not be unlimited, and why the organism should not therefore be
endowed with everlasting life. In the same manner, from a physiological
point of view, we might admit that we can see no reason why the
functions of the organism should ever cease.

It is only from the point of view of utility that we can understand the
necessity of death. The same arguments which were employed to explain
the necessity for as short a life as possible, will with but slight
modification serve to explain the common necessity of death[4].

Let us imagine that one of the higher animals became immortal; it then
becomes perfectly obvious that it would cease to be of value to the
species to which it belonged. Suppose that such an immortal individual
could escape all fatal accidents, through infinite time,—a supposition
which is of course hardly conceivable. The individual would
nevertheless be unable to avoid, from time to time, slight injuries to
one or another part of its body. The injured parts could not regain
their former integrity, and thus the longer the individual lived, the
more defective and crippled it would become, and the less perfectly
would it fulfil the purpose of its species. Individuals are injured by
the operation of external forces, and for this reason alone it is
necessary that new and perfect individuals should continually arise and
take their place, and this necessity would remain even if the
individuals possessed the power of living eternally.

From this follows, on the one hand, the necessity of reproduction, and,
on the other, the utility of death. Worn-out individuals are not only
valueless to the species, but they are even harmful, for they take the
place of those which are sound. Hence by the operation of natural
selection, the life of our hypothetically immortal individual would be
shortened by the amount which was useless to the species. It would be
reduced to a length which would afford the most favourable conditions
for the existence of as large a number as possible of vigorous
individuals, at the same time.

If by these considerations death is shown to be a beneficial
occurrence, it by no means follows that it is to be solely accounted
for on grounds of utility. Death might also depend upon causes which
lie in the nature of life itself. The floating of ice upon water seems
to us to be a useful arrangement, although the fact that it does float
depends upon its molecular structure and not upon the fact that its
doing so is of any advantage to us. In like manner the necessity of
death has been hitherto explained as due to causes which are inherent
in organic nature, and not to the fact that it may be advantageous.

I do not however believe in the validity of this explanation; I
consider that death is not a primary necessity, but that it has been
secondarily acquired as an adaptation. I believe that life is endowed
with a fixed duration, not because it is contrary to its nature to be
unlimited, but because the unlimited existence of individuals would be
a luxury without any corresponding advantage. The above-mentioned
hypothesis upon the origin and necessity of death leads me to believe
that the organism did not finally cease to renew the worn-out cell
material because the nature of the cells did not permit them to
multiply indefinitely, but because the power of multiplying
indefinitely was lost when it ceased to be of use.

I consider that this view, if not exactly proved, can at any rate be
rendered extremely probable.

It is useless to object that man (or any of the higher animals) dies
from the physical necessity of his nature, just as the specific gravity
of ice results from its physical nature. I am quite ready to admit that
this is the case. John Hunter, supported by his experiments on
_anabiosis_, hoped to prolong the life of man indefinitely by alternate
freezing and thawing; and the Veronese Colonel Aless. Guaguino made his
contemporaries believe that a race of men existed in Russia, of which
the individuals died regularly every year on the 27th of November, and
returned to life on the 24th of the following April. There cannot
however be the least doubt, that the higher organisms, as they are now
constructed, contain within themselves the germs of death. The question
however arises as to how this has come to pass; and I reply that death
is to be looked upon as an occurrence which is advantageous to the
species as a concession to the outer conditions of life, and not as an
absolute necessity, essentially inherent in life itself.

Death, that is the end of life, is by no means, as is usually assumed,
an attribute of all organisms. An immense number of low organisms do
not die, although they are easily destroyed, being killed by heat,
poisons, &c. As long, however, as those conditions which are necessary
for their life are fulfilled, they continue to live, and they thus
carry the potentiality of unending life in themselves. I am speaking
not only of the Amoebae and the low unicellular Algae, but also of far
more highly organized unicellular animals, such as the Infusoria.

The process of fission in the Amoeba has been recently much discussed,
and I am well aware that the life of the individual is generally
believed to come to an end with the division which gives rise to two
new individuals, as if death and reproduction were the same thing. But
this process cannot be truly called death. Where is the dead body? what
is it that dies? Nothing dies; the body of the animal only divides into
two similar parts, possessing the same constitution. Each of these
parts is exactly like its parent, lives in the same manner, and finally
also divides into two halves. As far as these organisms are concerned,
death can only be spoken of in the most figurative sense.

There are no grounds for the assumption that the two halves of an
Amoeba are differently constituted internally, so that after a time one
of them will die while the other continues to live. Such an idea is
disproved by a recently discovered fact. It has been noticed in
_Euglypha_ (one of the Foraminifera) and in other low animals of the
same group, that when division is almost complete, and the two halves
are only connected by a short strand, the protoplasm of both parts
begins to circulate, and for some time passes backwards and forwards
between the two halves. A complete mingling of the whole substance of
the animal and a resulting identity in the constitution of each half is
thus brought about before the final separation [See Note 10].

The objection might perhaps be raised that, if the parent animal does
not exactly die, it nevertheless disappears as an individual. I cannot
however let this pass unless it is also maintained that the man of
to-day is no longer the same individual as the boy of twenty years ago.
In the growth of man, neither structure nor the components of structure
remain precisely the same; the material is continually changing. If we
can imagine an Amoeba endowed with self-consciousness, it might think
before dividing ‘I will give birth to a daughter,’ and I have no doubt
that each half would regard the other as the daughter, and would
consider itself to be the original parent. We cannot however appeal to
this criterion of personality in the Amoeba, but there is nevertheless
a criterion which seems to me to decide the matter: I refer to the
continuity of life in the same form.

Now if numerous organisms, endowed with the potentiality of
never-ending life, have real existence, the question arises as to
whether the fact can be understood from the point of view of utility.
If death has been shown to be a necessary adaptation for the higher
organisms, why should it not be so for the lower also? Are they not
decimated by enemies? are they not often imperfect? are they not worn
out by contact with the external world? Although they are certainly
destroyed by other animals, there is nothing comparable to that
deterioration of the body which takes place in the higher organisms.
Unicellular animals are too simply constructed for this to be possible.
If an infusorian is injured by the loss of some part of its body, it
may often recover its former integrity, but if the injury is too great
it dies. The alternative is always perfect integrity or complete
destruction.

We may now leave this part of the subject, for it is obvious that
normal death, that is to say, death which arises from internal causes,
is an impossibility among these lower organisms. In those species at
any rate in which fission is accompanied by a circulation of the
protoplasm of the parent, the two halves must possess the same
qualities. Since one of them is endowed with a potentiality for
unending life, and must be so endowed if the species is to persist, it
is clear that the other exactly similar half must be endowed with equal
potentiality.

Let us now consider how it happened that the multicellular animals and
plants, which arose from unicellular forms of life, came to lose this
power of living for ever.

The answer to this question is closely bound up with the principle of
division of labour which appeared among multicellular organisms at a
very early stage, and which has gradually led to the production of
greater and greater complexity in their structure.

The first multicellular organism was probably a cluster of similar
cells, but these units soon lost their original homogeneity. As the
result of mere relative position, some of the cells were especially
fitted to provide for the nutrition of the colony, while others
undertook the work of reproduction. Hence the single group would come
to be divided into two groups of cells, which may be called somatic and
reproductive—the cells of the body as opposed to those which are
concerned with reproduction. This differentiation was not at first
absolute, and indeed it is not always so to-day. Among the lower
Metazoa, such as the polypes, the capacity for reproduction still
exists to such a degree in the somatic cells, that a small number of
them are able to give rise to a new organism,—in fact new individuals
are normally produced by means of so-called buds. Furthermore, it is
well known that many of the higher animals have retained considerable
powers of regeneration; the salamander can replace its lost tail or
foot, and the snail can reproduce its horns, eyes, etc.

As the complexity of the Metazoan body increased, the two groups of
cells became more sharply separated from each other. Very soon the
somatic cells surpassed the reproductive in number, and during this
increase they became more and more broken up by the principle of the
division of labour into sharply separated systems of tissues. As these
changes took place, the power of reproducing large parts of the
organism was lost, while the power of reproducing the whole individual
became concentrated in the reproductive cells alone.

But it does not therefore follow that the somatic cells were compelled
to lose the power of unlimited cell-production, although in accordance
with the law of heredity, they could only give rise to cells which
resembled themselves, and belonged to the same differentiated
histological system. But as the fact of normal death seems to teach us
that they have lost even this power, the causes of the loss must be
sought outside the organism, that is to say, in the external conditions
of life; and we have already seen that death can be very well explained
as a secondarily acquired adaptation. The reproductive cells cannot
lose the capacity for unlimited reproduction, or the species to which
they belong would suffer extinction. But the somatic cells have lost
this power to a gradually increasing extent, so that at length they
became restricted to a fixed, though perhaps very large number of
cell-generations. This restriction, which implies the continual influx
of new individuals, has been explained above as a result of the
impossibility of entirely protecting the individual from accidents, and
from the deterioration which follows them. Normal death could not take
place among unicellular organisms, because the individual and the
reproductive cell are one and the same: on the other hand, normal death
is possible, and as we see, has made its appearance, among
multicellular organisms in which the somatic and reproductive cells are
distinct.

I have endeavoured to explain death as the result of restriction in the
powers of reproduction possessed by the somatic cells, and I have
suggested that such restriction may conceivably follow from a
limitation in the number of cell-generations possible for the cells of
each organ and tissue. I am unable to indicate the molecular and
chemical properties of the cell upon which the duration of its power of
reproduction depends: to ask this is to demand an explanation of the
nature of heredity—a problem the solution of which may still occupy
many generations of scientists. At present we can hardly venture to
propose any explanation of the real nature of heredity.

But the question must be answered as to whether the kind and degree of
reproductive power resides in the nature of the cell itself, or in any
way depends upon the quality of its nutriment.

Virchow, in his ‘Cellular Pathology,’ has remarked that the cells are
not only nourished, but that they actively supply themselves with food.
If therefore the internal condition of the cell decides whether it
shall accept or reject the nutriment which is offered, it becomes
conceivable that all cells may possess the power of refusing to absorb
nutriment, and therefore of ceasing to undergo further division.

Modern embryology affords us many proofs, in the segmentation of the
ovum, and in the subsequent developmental changes, that the causes of
the different forms of reproductive activity witnessed in cells lie in
the essential nature of the cells themselves. Why does the segmentation
of one half of certain eggs proceed twice as rapidly as that of the
other half? why do the cells of the ectoderm divide so much more
quickly than those of the endoderm? Why does not only the rate, but
also the number of cells produced (so far as we can follow them) always
remain the same? Why does the multiplication of cells in every part of
the blastoderm take place with the exact amount of energy and rapidity
necessary to produce the various elevations, folds, invaginations,
etc., in which the different organs and tissues have their origin, and
from which finally the organism itself arises? There can be no doubt
that the causes of all these phenomena lie within the cells themselves;
that in the ovum and the cells which are immediately derived from it,
there exists a tendency towards a certain determined (I might almost
say specific) mode and energy of cell-multiplication. And why should we
regard this inherited tendency as confined to the building up of the
embryo? why should it not also exist in the young, and later in the
mature animal? The phenomena of heredity which make their appearance
even in old age afford us proofs that a tendency towards a certain mode
of cell-multiplication continues to regulate the growth of the organism
during the whole of its life.

The above-mentioned considerations show us that the degree of
reproductive activity present in the tissues is regulated by internal
causes while the natural death of an organism is the termination—the
hereditary limitation—of the process of cell-division, which began in
the segmentation of the ovum.

Allow me to suggest a further consideration which may be compared with
the former. The organism is not only limited in time, but also in
space: it not only lives for a limited period, but it can only attain a
limited size. Many animals grow to their full size long before their
natural end: and although many fishes, reptiles, and lower animals are
said to grow during the whole of their life, we do not mean by this
that they possess the power of unlimited growth any more than that of
unlimited life. There is everywhere a maximum size, which, as far as
our experience goes, is never surpassed. The mosquito never reaches the
size of an elephant, nor the elephant that of a whale.

Upon what does this depend? Is there any external obstacle to growth?
Or is the limitation entirely imposed from within?

Perhaps you may answer, that there is an established relation between
the increase of surface and mass, and it cannot be denied that these
relations do largely determine the size of the body. A beetle could
never reach the size of an elephant, because, constituted as it is, it
would be incapable of existence if it attained such dimensions. But
nevertheless the relations between surface and mass do not form the
only reason why any given individual does not exceed the average size
of its species. Each individual does not strive to grow to the largest
possible size, until the absorption from its digestive area becomes
insufficient for its mass; but it ceases to grow because its cells
cannot be sufficiently nourished in consequence of its increased size.
The giants which occasionally appear in the human species prove that
the plan upon which man is constructed can also be carried out on a
scale which is far larger than the normal one. If the size of the body
chiefly depends upon amount of nutriment, it would be possible to make
giants and dwarfs at will. But we know, on the contrary, that the size
of the body is hereditary in families to a very marked extent; in fact
so much so that the size of an individual depends chiefly upon
heredity, and not upon amount of food.

These observations point to the conclusion that the size of the
individual is in reality pre-determined, and that it is potentially
contained in the egg from which the individual developes.

We know further that the growth of the individual depends chiefly upon
the multiplication of cells and only to a slight extent upon the growth
of single cells. It is therefore clear that a limit of growth is
imposed by a limitation in the processes by which cells are increased,
both as regards the number of cells produced and the rate at which they
are formed. How could we otherwise explain the fact that an animal
ceases to grow long before it has reached the physiologically
attainable maximum of its species, without at the same time suffering
any loss of vital energy?

In many cases at least, the most important duty of an organism, viz.
reproduction, follows upon the attainment of full size—a fact which
induced Johannes Müller to reject the prevailing hypothesis which
explained the death of animals as due to ‘the influences of the
inorganic environment, which gradually wear away the life of the
individual.’ He argued that, if this were the case, ‘the organic energy
of an individual would steadily decrease from the beginning,’ while the
facts indicate that this is not so[5].

If it is further asked why the egg should give rise to a fixed number
of cell-generations, although perhaps a number which varies widely
within certain limits, we may now refer to the operation of natural
selection upon the relation of surface to mass, and upon other
physiological necessities which are peculiar to the species. Because a
certain size is the most favourable for a certain plan of organization,
the process of natural selection determined that such a size should be
within certain variable limits, characteristic of each species. This
size is then transmitted from generation to generation, for when once
established as normal for the species, the most favourable size is
potentially present in the reproductive cell from which each individual
is developed.

If this conclusion holds, and I believe that no essential objection can
be raised against it, then we have in the limitation in space a process
which is exactly analogous to the limitation in time, which we have
already considered. The latter limitation—the duration of life—also
depends upon the multiplication of cells, the rapid increase of which
first gave rise to the characteristic form of the mature body, and then
continued at a slower rate. In the mature animal, cell-reproduction
still goes on, but it no longer exceeds the waste; for some time it
just compensates for loss, and then begins to decline. The waste is not
compensated for, the tissues perform their functions incompletely, and
thus the way for death is prepared, until its final appearance by one
of the three great _Atria mortis_.

I admit that facts are still wanting upon which to base this
hypothesis. It is a pure supposition that senile changes are due to a
deficient reproduction of cells: at the same time this supposition
gains in probability when we are enabled to reduce the limitations of
the organism in both time and space to one and the same principle. It
cannot however be asserted under any circumstances that it is a pure
supposition that the ovum possesses a capacity for cell-multiplication
which is limited both as to numbers produced and rate of production.
The fact that each species maintains an average size is a sufficient
proof of the truth of this conclusion.

Hitherto I have only spoken of animals and have hardly mentioned
plants. I should not have been able to consider them at all, had it not
happened that a work of Hildebrand’s [See Note 12] has recently
appeared, which has, for the first time, provided us with exact
observations on the duration of plant-life.

The chief results obtained by this author agree very well with the view
which I have brought before you to-day. Hildebrand shows that the
duration of life in plants also is by no means completely fixed, and
that it may be very considerably altered through the agency of the
external conditions of life. He shows that, in course of time, and
under changed conditions of life, an annual plant may become perennial,
or _vice versa_. The external factors which influence the duration of
life are here however essentially different, as indeed we expect them
to be, when we remember the very different conditions under which the
animal and vegetable kingdoms exist. During the life of animals the
destruction of mature individuals plays a most important part, but the
existence of the mature plant is fairly well secured; their chief
period of destruction is during youth, and this fact has a direct
influence upon the degree of fertility, but not upon the duration of
life. Climatic considerations, especially the periodical changes of
summer and winter, or wet and dry seasons, are here of greater
importance.

It must then be admitted that the dependence of the duration of life
upon the external conditions of existence is alike common to plants and
animals. In both kingdoms the high multicellular forms with
well-differentiated organs contain the germs of death, while the low
unicellular organisms are potentially immortal. Furthermore, an undying
succession of reproductive cells is possessed by all the higher forms,
although this may be but poor consolation to the conscious individual
which perishes. Johannes Müller is therefore right, when in the
sentence quoted at the beginning of my lecture, he speaks of an
‘appearance of immortality’ which passes from each individual into that
which succeeds it. That which remains over, that which persists, is not
the individual itself,—not the complex aggregate of cells which is
conscious of itself,—but an individuality which is outside its
consciousness, and of a low order,—an individuality which is made up of
a single cell, which arises from the conscious individual. I might here
conclude, but I wish first, in a few words, to protect myself against a
possible misunderstanding.

I have repeatedly spoken of immortality, first of the unicellular
organism, and secondly of the reproductive cell. By this word I have
merely intended to imply a duration of time which appears to be endless
to our human faculties. I have no wish to enter into the question of
the cosmic or telluric origin of life on the earth. An answer to this
question will at once decide whether the power of reproduction
possessed by these cells is in reality eternal or only immensely
prolonged, for that which is without beginning is, and must be, without
end.

The supposition of a cosmic origin of life can only assist us if by its
means we can altogether dispense with any theory of spontaneous
generation. The mere shifting of the origin of life to some other
far-off world cannot in any way help us. A truly cosmic origin in its
widest significance will rigidly limit us to the statement—_omne vivum
e vivo_—to the idea that life can only arise from life, and has always
so arisen,—to the conclusion that organic beings are eternal like
matter itself.

Experience cannot help us to decide this question; we do not know
whether spontaneous generation was the commencement of life on the
earth, nor have we any direct evidence for the idea that the process of
development of the living world carries the end within itself, or for
the converse idea that the end can only be brought about by means of
some external force.

I admit that spontaneous generation, in spite of all vain efforts to
demonstrate it, remains for me a logical necessity. We cannot regard
organic and inorganic matter as independent of each other and both
eternal, for organic matter is continually passing, without residuum,
into the inorganic. If the eternal and indestructible are alone without
beginning, then the non-eternal and destructible must have had a
beginning. But the organic world is certainly not eternal and
indestructible in that absolute sense in which we apply these terms to
matter itself. We can, indeed, kill all organic beings and thus render
them inorganic at will. But these changes are not the same as those
which we induce in a piece of chalk by pouring sulphuric acid upon it;
in this ease we only change the form, and the inorganic matter remains.
But when we pour sulphuric acid upon a worm, or when we burn an oak
tree, these organisms are not changed into some other animal and tree,
but they disappear entirely as organized beings and are resolved into
inorganic elements. But that which can be completely resolved into
inorganic matter must have also arisen from it, and must owe its
ultimate foundation to it. The organic might be considered eternal if
we could only destroy its form, but not its nature.

It therefore follows that the organic world must once have arisen, and
further that it will at some time come to an end. Hence we must speak
of the eternal duration of unicellular organisms and of reproductive
cells in the Metazoa and Metaphyta in that particular sense which
signifies, when measured by our standards, an immensely long time.

Yet who can maintain that he has discovered the right answer to this
important question? And even though the discovery were made, can any
one believe that by its means the problem of life would be solved? If
it were established that spontaneous generation did actually occur, a
new question at once arises as to the conditions under which the
occurrence became possible. How can we conceive that dead inorganic
matter could have come together in such a manner as to form living
protoplasm, that wonderful and complex substance which absorbs foreign
material and changes it into its own substance, in other words grows
and multiplies?

And so, in discussing this question of life and death, we come at
last—as in all provinces of human research—upon problems which appear
to us to be, at least for the present, insoluble. In fact it is the
quest after perfected truth, not its possession, that falls to our lot,
that gladdens us, fills up the measure of our life, nay! hallows it.

                               APPENDIX.

               Note 1. The Duration of Life among Birds.

There is less exact knowledge upon this subject than we might expect,
considering the existing number of ornithologists and ornithological
societies with their numerous publications. It has neither been
possible nor necessary for my purpose to look up all the
widely-scattered references which are to be found upon the subject.
Many of these are doubtless unknown to me; for we are still in want of
a compilation of accurately determined observations in this department
of zoology. I print the few facts which I have been able to collect, as
a slight contribution towards such a compilation.

                  *       *       *       *       *

Small singing birds live from eight to eighteen years: the nightingale,
in captivity, eight years, but longer according to some writers: the
blackbird, in captivity, twelve years, but both these birds live longer
in the natural state. A ‘half-bred nightingale built its nest for nine
consecutive years in the same garden’ (Naumann, ‘Vögel Deutschlands,’
p. 76).

Canary birds in captivity attain an age of twelve to fifteen years (l.
c., p. 76).

Ravens have lived for almost a hundred years in captivity (l. c., Bd.
I. p. 125).

Magpies in captivity live twenty years, and, ‘without doubt,’ much
longer in the natural state (l. c., p. 346).

Parrots ‘in captivity have reached upwards of a hundred years’ (l. c.,
p. 125).

A single instance of the cuckoo (alluded to in the text) is mentioned
by Naumann as reaching the age of thirty-two years (l. c., p. 76).

Fowls live ten to twenty years, the golden pheasant fifteen years, the
turkey sixteen years, and the pigeon ten years (Oken, ‘Naturgeschichte,
Vögel,’ p. 387).

A golden eagle which ‘died at Vienna in the year 1719, had been
captured 104 years previously’ (Brehm, ‘Leben der Vögel,’ p. 72).

A falcon (species not mentioned) is said to have attained an age of 162
years (Knauer, ‘Der Naturhistoriker,’ Vienna, 1880).

A white-headed vulture which was taken in 1706 died in the Zoological
Gardens at Vienna (Schönbrunn) in 1824, thus living 118 years in
captivity (l. c.).

The example of the bearded vulture, mentioned in the text, is quoted
from Schinz’s ‘Vögel der Schweiz,’ p. 196.

The wild goose must live for upwards of 100 years, according to Naumann
(l. c., p. 127). The proof of this is not, however, forthcoming. A wild
goose which had been wounded reached its eighteenth year in captivity.

Swans are said to have lived 300 years(?), (Naumann, l. c., p. 127).

It is evident that observations upon the duration of life in wild birds
can only rarely be made, and that they are usually the result of chance
and cannot be verified. It is on this account all the more to be
desired that every ascertained fact should be collected.

If the long life of birds has been correctly interpreted as
compensation for their feeble fertility and for the great mortality of
their young, it will be possible to estimate the length of life in a
species, without direct observation, if we only know its fertility and
the percentage of individuals destroyed. This percentage can, however,
at best, be known only as an average. If we consider, for example, the
enormous number of sea birds which breed in summer on the rocks and
cliffs of the northern seas, and if we remember that the majority of
these birds lay but one, or at most two eggs yearly, and that their
young are exposed to very many destructive agencies, we are forced to
the conclusion that they must possess a very long life, so that the
breeding period may be many times repeated. Their number does not
diminish. Year after year countless numbers of these birds cover the
rocks, from summit to sea line; millions of them rest there, and rise
in the air like a thick cloud whenever they are disturbed. Even in
those localities which are every year visited by man in order to effect
their capture, the number does not appear to decrease, unless the birds
are disturbed and are therefore prompted to seek other breeding-places.
From the small island of St. Kilda, off Scotland, 20,000 young gannets
(_Sula_) and an immense number of eggs are annually collected; and
although this bird only lays a single egg yearly and takes four years
to attain maturity, the numbers do not diminish[6]. 30,000 sea-gulls’
eggs and 20,000 terns’ eggs are yearly exported from the
breeding-places on the island of Sylt, but in this case it appears that
a systematic disturbance of the birds is avoided by the collectors, and
no decrease in their numbers has yet taken place[7]. The destruction of
northern birds is not only caused by man, but also by various
predaceous mammals and birds. Indeed the dense mass of birds which
throng the cliffs is a cause of destruction to many of the young and to
the eggs, which are pushed over the edge of the rocks. According to
Brehm the foot of these cliffs is ‘always covered with blood and the
dead bodies of fledglings.’

Such birds must attain a great age or they would have been exterminated
long ago: the minimum duration of life necessary for the maintenance of
the species must in their case be a very high one.

              Note 2. The Duration of Life among Mammals.

The statements upon this subject in the text are taken from many
sources; from Giebel’s ‘Säugethiere,’ from Oken’s ‘Naturgeschichte,’
from Brehm’s ‘Illustrirtem Thierleben,’ and from an essay of Knauer in
the ‘Naturhistoriker,’ Vienna, 1880.

           Note 3. The Duration of Life among Mature Insects.

A short statement of the best established facts which I have been able
to find is given below. I have omitted the lengthening of imaginal life
which is due to hybernation in certain species. In almost all orders of
insects there are certain species which emerge from the pupa in the
autumn, but which first reproduce in the following spring. The time
spent in the torpid condition during winter cannot of course be
reckoned with the active life of the species, for its vital activity is
either entirely suspended for a time by freezing (_Anabiosis_:
Preyer[8]), or it is at any rate never more than a _vita minima_, with
a reduction of assimilation to its lowest point.

The following account does not make any claim to contain all or even
most of the facts scattered through the enormous mass of entomological
literature, and much less all that is privately known by individual
entomologists. It must therefore be looked upon as merely a first
attempt, a nucleus, around which the principal facts can be gradually
collected. It is unnecessary to give any special information as to the
duration of larval life, for numerous and exact observations upon this
part of the subject are contained in all entomological works.

                             I. Orthoptera.

Gryllotalpa. The eggs are laid in June or July, and the young are
hatched in from two to three weeks; they live through the winter, and
become sexually mature in the following May or June. ‘When the female
has deposited her eggs, her body collapses, and afterwards she does not
survive much longer than a month.’ ‘According as the females are
younger or older, they live a longer or shorter life, and hence some
females are even found in the autumn’ (Rösel, ‘Insektenbelustigungen,’
Bd. II. p. 92). Rösel believes that the female watches the eggs until
they are hatched, and this explains the fact that she outlives the
process of oviposition by about a month. It is not stated whether the
males die at an earlier period.

_Gryllus campestris_ becomes sexually mature in May, and sings from
June till October, ‘when they all die’ (Oken, ‘Naturgeschichte,’ Bd.
II. Abth. iii. p. 1527). It is hardly probable that any single
individual lives for the whole summer; probably, as in the case of
_Gryllotalpa_, the end of the life of those individuals which first
become mature, overlaps the beginning of the life of others which reach
maturity at a later date.

_Locusta viridissima_ and _L. verrucivora_ are mature at the end of
August; they lay their eggs in the earth during the first half of
September and then die. It is probable that the females do not live for
more than four weeks in the mature state. It is not known whether the
males of this or other species of locusts live for a shorter period.

I have found _Locusta cantans_ in plenty, from the beginning of
September to the end of the month. In captivity they die after
depositing their eggs: the males are probably more short-lived, for
towards the middle and end of September they are much less plentiful
than the females.

_Acridium migratorium_ ‘dies after the eggs are laid’ (Oken,
‘Naturgeschichte’).

The male _Termes_ probably live for a short time only, although exact
observations upon the point are wanting. The females ‘seem sometimes to
live four or five years,’ as I gather from a letter from Dr. Hagen, of
Cambridge, Mass., U.S.A.

_Ephemeridae._ Rösel, speaking of _Ephemera vulgata_
(‘Insektenbelustigungen,’ Bd. II. der Wasserinsekten, 2^{te} Klasse, p.
60 et seq.), says:—‘Their flight commences at sunset, and comes to an
end before midnight, when the dew begins to fall.’ ‘The pairing
generally takes place at night and lasts but a short time. As soon as
the insects have shed their last skin, in the afternoon or evening,
they fly about in thousands, and pair almost immediately; but by the
next day they are all dead. They continue to emerge for many days, so
that when yesterday’s swarm is dead, to-day a new swarm is seen
emerging from the water towards the evening.’ ‘They not only drop their
eggs in the water, but wherever they may happen to be,—on trees,
bushes, or the earth. Birds, trout and other fish lie in wait for them.’

Dr. Hagen writes to me—‘It is only in certain species that life is so
short. The female _Palingenia_ does not live long enough to complete
the last moult of the sub-imago. I believe that a female imago has
never been seen. The male imago, often half in its sub-imago skin,
fertilizes the female sub-imago and immediately the contents of both
ovaries are extruded, and the insect dies. It is quite possible that
the eggs pass out by rupturing the abdominal segments.’

_Libellula._ All dragon-flies live in the imago condition for some
weeks; at first they are not capable of reproduction, but after a few
days they pair.

_Lepisma saccharina._ An individual lived for two years in a pill-box,
without any food except perhaps a little _Lycopodium_ dust[9].

                            II. Neuroptera.

_Phryganids_ ‘live in the imago stage for at least a week and probably
longer, apparently without taking food’ (letter from Dr. Hagen).

According to the latest researches _Phrygane grandis_[10] never
contains food in its alimentary canal, but only air, although it
contains the latter in such quantities that the anterior end of the
chylific ventricle is dilated by it.

                           III. Strepsiptera.

The larva requires for its development a rather shorter time than that
which is necessary for the grub of the bee into the body of which it
has bored. The pupa stage lasts eight to ten days. The male, which
flies about in a most impetuous manner, lives only two to three hours,
while the female lives for some days. Possibly the pairing does not
take place until the female is two to three days old. The viviparous
female seems to produce young only once in a lifetime, and then dies:
it is at present uncertain whether she also produces young
parthenogenetically (cf. Siebold, ‘Ueber Paedogenesis der
Strepsipteren,’ Zeitschr. f. Wissensch. Zool., Band. XX, 1870).

                             IV. Hemiptera.

_Aphis._ Bonnet (‘Observations sur les Pucerons,’ Paris, 1745) had a
parthenogenetic female of _Aphis euonymi_ in his possession for
thirty-one days, from its birth, during which time it brought forth
ninety-five larvae. Gleichen kept a parthenogenetic female of _Aphis
mali_ fifteen to twenty-three days.

_Aphis foliorum ulmi._ The mother of a colony which leaves the egg in
May is 2‴ long at the end of July: it therefore lives for at least two
and a half months (De Geer, ‘Abhandlungen zur Geschichte der Insekten,’
1783, III. p. 53).

_Phylloxera vastatrix._ The males are merely ephemeral sexual
organisms, they have no proboscis and no alimentary canal, and die
immediately after fertilizing the female.

_Pemphigus terebinthi._ The male as well as the female sexual
individuals are wingless and without a proboscis; they cannot take food
and consequently live but a short time,—far shorter than the
parthenogenetic females of the same species (Derbès, ‘Note sur les
aphides du pistachier térébinthe,’ Ann. des sci. nat., Tom. XVII, 1872).

_Cicada._ In spite of the numerous and laborious descriptions of the
Cicadas which have appeared during the last two centuries, I can only
find precise statements as to the duration of life in the mature insect
in a single species. P. Kalm, writing upon the North American _Cicada
septemdecim_, which sometimes appears in countless numbers, states that
‘six weeks after (such a swarm had been first seen) they had all
disappeared.’ Hildreth puts the life of the female at from twenty to
twenty-five days. This agrees with the fact that the Cicada lays many
hundred eggs (Hildreth states a thousand); sixteen to twenty at a time
being inserted into a hole which is bored in wood, so that the female
takes some time to lay her eggs (Oken, ‘Naturgeschichte,’ 2^{ter} Bd.
3^{te} Abth. p. 1588 et seq.).

_Acanthia lectularia._ No observations have been made upon the bed bug
from which the normal length of its life can be ascertained, but many
statements tend to show that it is exceedingly long-lived, and this is
advantageous for a parasite of which the food (and consequently growth
and reproduction) is extremely precarious. They can endure starvation
for an astonishingly long period, and can survive the most intense
cold. Leunis (‘Zoologie,’ p. 659) mentions the case of a female which
was shut up in a box and forgotten: after six months’ starvation it was
found not only alive but surrounded by a circle of lively young ones.
Göze found bugs in the hangings of an old bed which had not been used
for six years: ‘they appeared white like paper.’ I have myself observed
a similar case, in which the starving animals were quite transparent.
De Geer placed some bugs in an unheated room in the cold winter of
1772, when the thermometer fell to -33°C: they passed the whole winter
in a state of torpidity, but revived in the following May. (De Geer,
Bd. III. p. 165, and Oken, ‘Naturgeschichte,’ 2^{ter} Bd. 3^{te} Abth. p.
1613.)

                              V. Diptera.

_Pulex irritans._ Oken says of the flea (‘Naturgeschichte,’ Bd. II.
Abth. 2, p. 759) that ‘death follows the deposition of the eggs in the
course of two or three days, even if the opportunity of sucking blood
is given them.’ The length of time which intervenes between the
emergence from the cocoon and fertilization or the deposition of eggs
is not stated.

_Sarcophaga carnaria._ The female fly dies ten to twelve hours after
the birth of the viviparous larvae; the time intervening between the
exit from the cocoon and the birth of the young is not given (Oken,
quoting Réaumur, ‘Mém. p. s. à l’hist. Insectes,’ Paris, 1740-48, IV).

_Musca domestica._ In the summer the common house-fly begins to lay
eggs eight days after leaving the cocoon: she then lays several times.
(See Gleichen, ‘Geschichte der gemeinen Stubenfliege,’ Nuremberg, 1764.)

_Eristalis tenax._ The larva of this large fly lives in liquid manure,
and has been described and figured by Réaumur as the rat-tailed larva.
I kept a female which had just emerged from the cocoon, from August
30th till October 4th, in a large gauze-covered glass vessel. The
insect soon learnt to move freely about in its prison, without
attempting to escape; it flew round in circles, with a characteristic
buzzing sound, and obtained abundant nourishment from a solution of
sugar, provided for it. From September 12th it ceased to fly about,
except when frightened, when it would fly a little way off. I thought
that it was about to die, but matters took an unexpected turn, and on
the 26th of September it laid a large packet of eggs, and again on the
29th of the same month another packet of similar size. The flight of
the animal had been probably impeded by the weight of the mass of ripe
eggs in its body. The deposition of eggs was probably considerably
retarded in this case, because fertilization had not taken place. The
fly died on the 4th of October, having thus lived for thirty-five days.
Unfortunately, I have been unable to make any experiments as to the
duration of life in the female when males are also present.

                            VI. Lepidoptera.

I am especially indebted to Mr. W. H. Edwards[11], of Coalburgh, W.
Virginia, and to Dr. Speyer, of Rhoden, for valuable letters relating
to this order.

The latter writes, speaking of the duration of life in imagos
generally:—‘It is, to my mind, improbable that any butterfly can live
as an imago for a twelvemonth. Specimens which have lived through the
winter are only rarely seen in August, even when the summer is late. A
worn specimen of _Vanessa cardui_ has, for instance, been found at this
time’ (‘Entomolog. Nachrichten,’ 1881, p. 146).

In answer to my question as to whether the fact that certain
Lepidoptera take no solid or liquid food, and are, in fact, without a
functional mouth, may be considered as evidence for an adaptation of
the length of life to the rapid deposition of eggs, Dr. Speyer
replies:—‘The wingless females of the _Psychidae_ do not seem to
possess a mouth, at any rate I cannot find one in _Psyche unicolor_
(_graminella_). They do not leave the case during life, and certainly
do not drink water. The same is true of the wingless female of
_Heterogynis_, and of _Orgyia ericae_, and probably of all the females
of the genus _Orgyia_; and as far as I can judge from cabinet
specimens, it is probably true of the males of _Heterogynis_ and
_Psyche_. I have never seen the day-flying _Saturnidae_, _Bombycidae_,
and other Lepidoptera with a rudimentary proboscis, settle in damp
places, or suck any moist substance, and I doubt if they would ever do
this. The sucking apparatus is probably deficient.’

In answer to my question as to whether the males of any species of
butterfly or moth are known to pass a life of different length from
that of the female, Dr. Speyer stated that he knew of no observations
on this point.

The following are the only instances of well-established direct
observations upon single individuals, in my possession[12]:—

_Pieris napi_, var. _bryoniae_ [male] and [female], captured on the
wing: lived in confinement ten days, and were then killed.

_Vanessa prorsa_ lived at most ten days in confinement.

_Vanessa urticae_ lived ten to thirteen days in confinement.

_Papilio ajax._ According to a letter from Mr. W. H. Edwards, the
female, when she leaves the pupa, contains unripe eggs in her body, and
lives for about six weeks—calculating from the first appearance of this
butterfly to the disappearance of the same generation[13]. The males
live longer, and continue to fly when very worn and exhausted. A worn
female is very seldom seen;—‘I believe the female does not live long
after laying her eggs, but this takes some days, and probably two
weeks.’

_Lycaena violacea._ According to Mr. Edwards, the first brood of this
species lives three to four weeks at the most.

_Smerinthus tiliae._ A female, which had just emerged from the pupa,
was caught on June 24th; on the 29th pairing took place; on the 1st of
July she laid about eighty eggs, and died the following day. She lived
nine days, taking no food during this period, and she only survived the
deposition of eggs by a single day.

_Macroglossa stellatarum._ A female, captured on the wing and already
fertilized, lived in confinement from June 28th to July 4th. During
this time she laid about eighty eggs, at intervals and singly; she then
disappeared, and must have died, although the body could not be found
among the grass at the bottom of the cage in which she was confined.

_Saturnia pyri._ A pair which quitted the cocoons on the 24th or 25th
of April, remained in coitu from the 26th until May 2nd—six or seven
days; the female then laid a number of eggs, and died.

_Psyche graminella._ The fertilized female lives some days, and the
unfertilized female over a week (Speyer).

_Solenobia triquetrella._ ‘The parthenogenetic form (I refer to the one
which I have shown to be parthenogenetic in Oken’s ‘Isis,’ 1846, p. 30)
lays a mass of eggs in the abandoned case, soon after emergence. The
oviposition causes her body to shrivel up, and some hours afterwards
she dies. The non-parthenogenetic female of the same species remains
for many days, waiting to be fertilized; if this does not occur, she
lives over a week.’ ‘The parthenogenetic female lives for hardly a day,
and the same is true of the parthenogenetic females of another species
of _Solenobia_’ (_S. inconspicuella_?). Letter from Dr. Speyer.

_Psyche calcella, O._ The males live a very short time; ‘those which
leave the cocoon in the evening are found dead on the following
morning, with their wings fallen off, at the bottom of their cage.’ Dr.
Speyer.

_Eupithecia_, sp. (_Geometridae_), ‘when well-fed, live for three to
four weeks in confinement; the males fertilize the females frequently,
and the latter continue to lay eggs when they are very feeble, and are
incapable of creeping or flying.’ Dr. Speyer.

The conclusions and speculations in the text seem to be sufficiently
supported from this short series of observations. There remains, as we
see, much to be done in this field, and it would well repay a
lepidopterist to undertake some exact observations upon the length of
life in different butterflies and moths, with reference to the
conditions of life—the mode of egg-laying, the degeneracy of the wings,
and of the external mouth-parts or the closure of the mouth itself. It
would be well to ascertain whether such closure does really take place,
as it undoubtedly does in certain plant-lice.

                            VII. Coleoptera.

_Melolontha vulgaris._ Cockchafers, which I kept in an airy cage with
fresh food and abundant moisture, did not in any case live longer than
thirty-nine days. One female only, out of a total number of forty-nine,
lived for this period; a second lived thirty-six days, a third
thirty-five, and a fourth and fifth twenty-four days; all the rest died
earlier. Of the males, only one lived as long as twenty-nine days.
These periods are less by some days than the true maximum duration of
life, for the beetles were captured in the field, and had lived for at
least a day; but the difference cannot be great, when we remember that
out of forty-nine beetles, only three females lived thirty-five to
thirty-nine days, and only one male twenty-nine days. Those that died
earlier had probably lived for some considerable time before being
caught.

Exact experiments with pupae which have survived the winter would show
whether the female really lives for ten days more than the male, or
whether the results of my experiment were merely accidental. I may add
that coitus frequently took place during the period of captivity. One
pair, observed in this condition on the 17th, separated in the evening;
they paired again on the morning of the 18th, and separated in the
middle of the day. Coitus took place between another pair on the 22nd,
and again on the 26th.

I watched the gradual approach of death in many individuals: some days
before it ensued, the insects became sluggish, ceased to fly and to
eat, and only crept a little way off when disturbed: they then fell to
the ground and remained motionless, apparently dead, but moved their
legs when irritated, and sometimes automatically. Death came on
gradually and imperceptibly; from time to time there was a slow
movement of the legs, and at last, after some hours, all signs of life
ceased.

In one case only I found bacteria present in great numbers in the blood
and tissues; in the other individuals which had recently died, the only
noticeable change was the unusual dryness of the tissues.

_Carabus auratus._ An experiment with an individual, caught on May
27th, gave the length of life at fourteen days; this is probably below
the average, since the beetles are found, in the wild state, from the
end of May until the beginning of July.

_Lucanus cervus._ Captured individuals, kept in confinement, and fed on
a solution of sugar, never lived longer than fourteen days, and as a
rule not so long. The beetles appear in June and July, and certainly
cannot live much over a month. As is the case with many beetles
appearing during certain months, the length of the individual life is
shorter than the period over which they are found. Accurate
information, especially as to any difference between the lengths of
life in the sexes, is not obtainable.

                  *       *       *       *       *

Isolated accounts of remarkably long lives among beetles are to be
found scattered throughout the literature of the subject. Dr. Hagen, of
Cambridge, Mass., has been kind enough to draw my attention to these,
and to send me some observations of his own.

_Cerambyx heros._ One individual lived in confinement from August until
the following year[14].

_Saperda carcharias._ An individual lived from the 5th of July until
the 24th of July of the next year[15].

_Buprestis splendens._ A living individual was removed from a desk
which had stood in a London counting-house for thirty years; from the
condition of the wood it was evident that the larva had been in it
before the desk was made[16].

_Blaps mortisaga._ One individual lived three months, and two others
three years.

_Blaps fatidica._ One individual which was left in a box and forgotten,
was found alive when the box was opened six years afterwards.

_Blaps obtusa._ One lived a year and a half in confinement.

_Eleodes grandis_ and _E. dentipes_. Eight of these beetles from
California were kept in confinement and without food for two years by
Dr. Gissler, of Brooklyn; they were then sent to Dr. Hagen who kept
them another year.

_Goliathus cacicus._ One individual lived in a hot-house for five
months.

In addition to these cases, Dr. Hagen writes to me: ‘Among the beetles
which live for more than a year,—_Blaps_, _Pasimachus_,
(_Carabidae_)—and among ants, almost thirty per cent. are found with
the cuticle worn out and cracked, and the powerful mandibles so greatly
worn down that species were formerly founded upon this point. The
mandibles are sometimes worn down to the hypodermis.’

From the data before me I am inclined to believe that in certain
beetles the normal length of life extends over some years, and this is
especially the case with the _Blapidae_. It seems probable that in
these cases another factor is present,—a _vita minima_, or apparent
death, a sinking of the vital processes to a minimum in consequence of
starvation, which we might call the hunger sleep, after the analogy of
winter sleep. The winter sleep is usually ascribed to cold alone, and
some insects certainly become so torpid that they appear to be dead
when the temperature is low. But cold does not affect all insects in
this way. Among bees, for example, the activity of the insects
diminishes to a marked extent at the beginning of winter, but if the
temperature continues to fall, they become active again, run about, and
as the bee-keepers say, ‘try to warm themselves by exercise’; by this
means they keep some life in them. If the frost is very severe, they
die. In the tropics the period of hibernation for many animals
coincides with the time of maximum heat and drought. This shows that
the organism can be brought into the condition of a _vita minima_ in
various ways, and it would not be at all remarkable if such a state
were induced in certain insects by hunger. Exact experiments however
are the only means by which such a suggestion can be tested, and I have
already commenced a series of experiments. The fact that certain
beetles live without food for many years (even six) can hardly be
explained on any other supposition, for these insects consume a fair
amount of food under normal conditions, and it is inconceivable that
they could live for years without food, if the metabolism were carried
on with its usual energy.

A very striking example, showing that longevity may be induced by the
lengthening of the period of reproductive activity, is communicated to
me by Dr. Adler in the following note: ‘Three years ago I accidentally
noticed that ovoviviparous development takes place in _Chrysomela
varians_,—a fact which I afterwards discovered had been already
described by another entomologist.

‘The egg passes through all the developmental stages in the ovary; when
these are completed the egg is laid, and a minute or two afterwards the
larva breaks through the egg-shell. In each division of the ovary the
eggs undergo development one at a time; it therefore follows that they
are laid at considerable intervals, so that a long life becomes
necessary in order to ensure the development of a sufficiently long
series of eggs. Hence it comes about that the females live a full year.
Among other species of _Chrysomela_ two generations succeed each other
in a year, and the duration of life in the individual varies from a few
months to half a year.’

                           VIII. Hymenoptera.

_Cynipidae._ I have been unable to find any accurate accounts of the
duration of life in the imagos of saw-flies or ichneumons; but on the
other hand I owe to the kindness of Dr. Adler, an excellent observer of
the _Cynipidae_, the precise accounts of that family which are in my
possession. I asked Dr. Adler the general question as to whether there
was any variation in the duration of life among the _Cynipidae_
corresponding to the conditions under which the deposition of eggs took
place; whether those species which lay many eggs, or of which the
oviposition is laborious and protracted, lived longer than those
species which lay relatively few eggs, or easily and quickly find the
suitable places in which to deposit them.

Dr. Adler fully confirmed my suppositions and supported them by the
following statements:—

‘The summer generation of _Neuroterus_ (_Spathegaster_) has the
shortest life of all _Cynipidae_. Whether captured or reared from the
galls I have only kept them alive on an average for three to four days.
In this generation the work of oviposition requires the shortest time
and the least expenditure of energy, for the eggs are simply laid on
the surface of a leaf. The number of eggs in the ovary is also smaller
than that of other species, averaging about 200. This form of _Cynips_
can easily lay 100 eggs a day.

‘The summer generation of _Dryophanta_ (_Spathegaster Taschenbergi_,
_verrucosus_, etc.) lives somewhat longer; I have kept them in
confinement for six to eight days. The oviposition requires a
considerable expenditure of time and strength, for the ovipositor has
to pierce the rather tough mid-rib or vein of a leaf. The number of
eggs in the ovary averages 300 to 400.

‘The summer generation of _Andricus_, which belongs to the extensive
genus _Aphilotrix_, have also a long life. I have kept the smaller
_Andricus_ (such as _A. nudus_, _A. cirratus_, _A. noduli_) alive for a
week, and the larger (_A. inflator_, _A. curvator_, _A. ramuli_) for
two weeks. The smaller species pierce the young buds when quite soft,
but the larger ones bore through the fully grown buds protected by
tough scales. The ovary of the former contains 400 to 500 eggs, that of
the latter over 600.

‘The agamic winter generations live much longer. The species of
_Neuroterus_ have the shortest life; they live for two weeks at the
outside; on the other hand, species of _Aphilotrix_ live quite four
weeks, and _Dryophanta_ and _Biorhiza_ even longer. I have kept
_Dryophanta scutellaris_ alive for three months. The number of eggs in
these agamic _Cynipidae_ is much larger: _Dryophanta_ and _Aphilotrix_
contain 1200 and _Neuroterus_ about 1000.’

It is evidently, therefore, a general rule that the duration of life is
directly proportional to the number of eggs and to the time and energy
expended in oviposition. It must of course be understood that, here as
in all other instances, these are not the only factors which determine
the duration of life, but many other factors, at present unknown, may
be in combination with them and assist in producing the result. For
example, it is very probable that the time of year at which the imagos
appear exerts some indirect influence. The long-lived _Biorhiza_
emerges from the gall in the middle of winter, and at once begins to
deposit eggs in the oak buds. Although the insect is not sensitive to
low temperature, for I have myself seen oviposition proceeding when the
thermometer stood at 5° R., yet very severe frost would certainly lead
to interruption and would cause the insect to shelter itself among dead
leaves on the ground. Such interruptions may be of long duration and
frequently repeated, so that the remarkably long life of this species
may perhaps be looked upon as an adaptation to its winter life.

_Ants._ _Lasius flavus_ lays its eggs in the autumn, and the young
larvae pass the winter in the nest. The males and females leave the
cocoons in June, and pair during July and August. The males fly out of
the nest with the females, but they do not return to it; ‘they die
shortly after pairing.’ It is also believed that the females do not
return to the nest, but found new colonies; this point is however one
of the most uncertain in the natural history of ants. On the other hand
it is quite certain that the female may live for years within the nest,
continuing to lay fertilized eggs. Old females are sometimes found in
the colony, with their jaws worn down to the hypodermis.

Breeding experiments confirm these statements. P. Huber[17] and Christ
have already put the life of the female at three to four years, and Sir
John Lubbock, who has been lately occupied with the natural history of
ants, was able to keep a female worker of _Formica sanguinea_ alive for
five years; and he has been kind enough to write and inform me that two
females of _Formica fusca_, which he captured in a wood together with
ten workers, in December 1874, are still alive (July 1881), so that
these insects live as imagos for six and a half years or more[18].

On the other hand, Sir John Lubbock never succeeded in keeping the
males ‘alive longer than a few weeks.’ Both the older and more recent
observers agree in stating that female ants, like queen bees, are
always protected as completely as possible from injury and danger. Dr.
A. Forel, whose thorough knowledge of Swiss ants is well known, writes
to me,—‘The female ants are only once fertilized, and are then tended
by the workers, being cleaned and fed in the middle of the nest: one
often finds them with only three legs, and with their chitinous armour
greatly worn. They never leave the centre of the nest, and their only
duty is to lay eggs.’

With regard to the workers, Forel believes that their constitution
would enable them to live as long as the females (as the experiments of
Lubbock also indicate), and the fact that in the wild state they
generally die sooner than the females is ‘certainly connected with the
fact that they are exposed to far greater dangers.’ The same relation
seems also to obtain among bees, but with them it has not been shown
that in confinement the workers live as long as the queens.

_Bees._ According to von Berlepsch[19] the queen may as an exception
live for five years, but as a rule survives only two or three years.
The workers always seem to live for a much shorter period, generally
less than a year. Direct experiments upon isolated or confined bees, or
upon marked individuals in the wild state, do not prove this, but the
statistics obtained by bee-keepers confirm the above. Every winter the
numbers in a hive diminish from 12,000-20,000 to 2000-3000. The queen
lays the largest number of eggs in the spring, and the workers which
die before the winter are replaced by those which emerge in the summer,
autumn or during a mild winter. The queen lays eggs at such a variable
rate throughout the year that the above-mentioned inequality in numbers
is explained. The workers do not often live for more than six to seven
months, and at the time of their greatest labour, (May to July), only
three months. An attempt to calculate the length of life of the workers
and drones by taking stock at the end of summer, gives six months for
the former and four months for the latter[20].

The drones do not as a rule live so long as four months, for they meet
with a violent death before the end of this period. The well-known
slaughter of the drones is not, according to the latest observations,
brought about directly by means of the stings of the workers, but by
these latter driving away the useless drones from the food so that they
perish of starvation.

_Wasps._ It is interesting that among these near relations of the bees,
the life of the female should be much shorter, corresponding to the
much lower degree of specialization found in the colonies. The females
of _Polistes gallica_ and of _Vespa_ not only lay eggs but take part in
building the cells and in collecting food; they are therefore obliged
to use all parts of the body more actively and especially the wings,
and are exposed to greater danger from enemies.

It is well known from Leuckart’s observations, that the so-called
‘workers’ of _Polistes gallica_ and _Bombus_ are not arrested females
like the workers of a bee-hive, but are females which although
certainly smaller, are in every way capable of being fertilized and of
reproduction. Von Siebold has nevertheless proved that they are not
fertilized, but reproduce parthenogenetically.

The fertilized female which survives the winter, commences to found a
colony at the beginning of May: the larvæ, which hatch from the first
eggs, which are about fifteen in number, become pupæ at the beginning
of June, and the imagos appear towards the end of the same month. These
are all small ‘workers,’ and they perform such good service in tending
the second brood, that the latter attain the size of the female which
founded the colony; only differing from her in the perfect condition of
their wings, for by this time her wings are greatly worn away.

The males appear at the beginning of July; their spermatozoa are mature
in August, and pairing then takes place with certain ‘special females
which require fertilization’ which have in the meantime emerged from
their cocoons. These are the females which live through the winter and
found new colonies in the following spring. The old females of the
previous winter die, and do not live beyond the summer at the beginning
of which they founded colonies. At the first appearance of frost, the
young fertilized females seek out winter quarters; the males which
never survive the winter, do not take this course, but perish in
October. The parthenogenetic females, which remain in the nest during
the nuptial flight, also perish.

The males of _Polistes gallica_ do not live longer than three
months—from July to the beginning of October; the parthenogenetic
females live a fortnight longer at the outside—from the middle of June
to October, but the later generations have a shorter life. The sexual
females alone live for about a year, including the winter sleep.

A similar course of events takes place in the genus _Vespa_. In both
these genera the possibility of reproduction is not restricted to a
single female in the nest, but is shared by a number of females. In the
genus _Apis_ alone is the division of labour complete, so that only a
single female (the queen) is at any one time capable of reproduction, a
power which differentiates it from the sterile workers.

       Note 4. The Duration of Life of the Lower Marine Animals.

I have only met with one definite statement in the literature of this
part of the subject. It concerns a sea anemone,—which is a solitary and
not a colonial form. The English zoologist Dalyell, in August, 1828,
removed an _Actinia mesembryanthemum_ from the sea and placed it in an
aquarium[21]. It was a very fine individual, although it had not quite
attained the largest size; and it must have been at least seven years
old, as proved by comparison with other individuals reared from the
egg. In the year 1848, it was about thirty years old, and in the twenty
years during which it had been in captivity it had produced 334 young
Actiniae. Prof. Dohrn, of Naples, tells me that this Actinia is still
living to-day, and is shown as a curiosity to those who visit the
Botanical Gardens in Edinburgh. It is now (1882) at least sixty-one
years old[22].

                    Note 5. The Duration of Life in
            Indigenous Terrestrial and Fresh-water Mollusca.

I am indebted to Herr Clessin—the celebrated student of our
mollusca—for some valuable notes upon our indigenous snails and
bivalves (_Lamellibranchiata_). I could not incorporate them in the
text, for a number of necessary details as to the conditions of life
are at present entirely unknown, or are at least only known in a very
fragmentary manner. No statistics as to the amount of destruction
suffered by the young are available, and even the number of eggs
produced annually is only known for a few species. I nevertheless
include Herr Clessin’s very interesting communications, as a
commencement to the life statistics of the Mollusca.

(1) ‘_Vitrinae_ are annual; the old animals die in the spring, after
having produced the spawn from which the young develope. These continue
to grow until the following spring.’

(2) ‘The _Succineae_ are mostly biennial; _Succinea putris_ probably
triennial. Fertilization takes place from June till the beginning of
August, and the young develope until the autumn. _Succinea Pfeifferi_
and _S. elegans_ live through the winter, and the fact is proved by
very distinct annual markings. Reproduction takes place in July and
August of the following year, and they die in the autumn. They continue
to grow until their death.’

(3) ‘The shells of our native species of _Pupa_, _Clausilia_, and
_Bulimus_ (with the exception of _Bulimus detritus_) show but faint
annual markings. They can hardly require more than two years for their
complete development. The great number of living individuals with
full-sized shells belonging to these genera, as compared with the
number which possess smaller shells, makes it probable that these
animals live in the mature condition longer than our other _Helicidae_.
I have always found full-sized shells present in at least two-thirds of
the individuals of these genera characterized by much-coiled shells—a
proportion which I have never seen among our larger _Helicidae_.
Nevertheless direct observations as to the length of life in the mature
condition are still wanting.’

(4) ‘The _Helicidae_ live from two to four years; _Helix sericea_, _H.
hispida_, two to three years; _H. hortensis_, _H. nemoralis_, _H.
arbustorum_, as a rule three years; _H. pomatia_ four years.
Fertilization is not in these species strictly confined to any one time
of year, but in the case of old animals takes place in the spring, as
soon as the winter sleep is over; while in the two-year-old animals it
also happens later in the summer.’

(5) ‘The _Hyalineae_ are mostly biennial: they seldom live three years,
and even in the largest species such an age is probably exceptional.
The smallest _Hyalineae_ and _Helicidae_ live at most two years. The
length of life is dependent upon the time at which the parents are
fertilized, for this decides whether the young begin to shift for
themselves early in the summer or later in the autumn, and so whether
the first year’s growth is large or small.’

(6) ‘The species of _Limnaeus_, _Planorbis_, and _Ancylus_ live two to
three years, that is they take two to three years to attain the full
size. _L. auricularis_ is mostly biennial, _L. palustris_ and _L.
pereger_ two to three years: I have found that the latter, in the
mountains at Oberstorf in the Bavarian Alps, may exceptionally attain
the age of four years, that is, it may possess three clearly defined
annual markings, whilst the specimens from the plain never showed more
than two.’

(7) ‘The _Paludinidae_ attain an age of three or four years.’

(8) ‘The smaller bivalves, _Pisidium_ and _Cyclas_, do not often live
for more than two years: the larger _Najadae_, on the other hand, often
live for more than ten years, and indeed they are not full grown until
they possess ten to fourteen annual markings. It is possible that
habitat may have great influence upon the length of life in this order.’

‘_Unio_ and _Anodonta_ become sexually mature in the third to the fifth
year.’

As far as I am aware but few statements exist upon the length of life
in marine mollusca, and these are for the most part very inexact. The
giant bivalve _Tridacna gigas_ must attain an age of 60 to 100
years[23]. All _Cephalopods_ live for at least over a year, and most of
them well over ten years; and the giant forms, sometimes mistaken for
‘sea-serpents,’ must require many decades in which to attain such a
remarkable size. L. Agassiz has determined the length of life in a
large sea snail, _Natica heros_, by sorting a great number of
individuals according to their sizes: he places it at 30 years[24].

I am glad to be able to communicate an observation made at the
Zoological Station at Naples upon the length of life in _Ascidians_.
The beautiful white _Cionea intestinalis_ has settled in great numbers
in an aquarium at the Station, and Professor Dohrn tells me that it
produces three generations annually, and that each individual lives for
about five months, and then reproduces itself and dies. External
conditions accounting for this early death have not been discovered.

It is known that the freshwater _Polyzoa_ are annual, but it is not
known whether the first individuals produced from a colony in the
spring, live for the whole summer. The length of life is also unknown
in single individuals of any marine Polyzoon.

Clessin’s accurate statements upon the freshwater Mollusca, previously
quoted, show that a surprisingly short length of life is the general
rule. Only those forms of which the large size requires that many years
shall elapse before the attainment of sexual maturity, live ten years
or over (_Unio_, _Anodonta_); indeed, our largest native snail (_Helix
pomatia_) only lives for four years, and many small species only one
year, or two years if the former time is insufficient to render them
sexually mature. These facts seem to indicate, as I think, that these
molluscs are exposed to great destruction in the adult state, indeed to
a greater extent than when they are young, or, at any rate, to an equal
extent. The facts appear to be the reverse of those found among birds.
The fertility is enormous; a single mussel contains several hundred
thousand eggs; the destruction of young as compared with the number of
eggs produced is distinctly smaller than in birds, therefore a much
shorter duration of the life of each mature individual is rendered
possible, and further becomes advantageous because the mature
individuals are exposed to severe destruction.

However it can only be vaguely suggested that this is the case, for
positive proofs are entirely absent. Perhaps the destruction of single
mature individuals does not play so important a part as the destruction
of their generative organs. The ravages of parasitic animals
(_Trematodes_) in the internal organs of snails and bivalves are well
known to zoologists. The ovaries of the latter are often entirely
filled with parasites, and such animals are then incapable of
reproduction.

Besides, molluscs have many enemies, which destroy them both on land
and in water. In the water,—fish, frogs, newts, ducks and other
water-fowl, and on land many birds, the hedgehog, toads, etc., largely
depend upon them for food.

If the principles developed in this essay apply to the freshwater
Mollusca, we must then infer that snails which maintain the mature
condition—the capability of reproduction—for one year, are in this
state more exposed to destruction from the attacks of enemies than
those species which remain sexually mature for two or three years, or
that the latter suffer from a greater proportional loss of eggs and
young.

            Note 6. Unequal Length of Life in the two Sexes.

This inequality is frequently found among insects. The males of the
remarkable little parasites infesting bees, the _Strepsiptera_, only
live for two to three hours in the mature condition, while the
wingless, maggot-like, female lives eight days: in this case,
therefore, the female lives sixty-four times as long as the male. The
explanation of these relations is obvious; a long life for the male
would be useless to the species, while the relatively long life of the
female is a necessity for the species, inasmuch as she is viviparous,
and must nourish her young until their birth.

Again, the male of _Phylloxera vastatrix_ lives for a much shorter
period than the female, and is devoid of proboscis and stomach, and
takes no food: it fertilizes the female as soon as the last skin has
been shed and then dies.

Insects are not the only animals among which we find inequality in the
length of life of the two sexes. Very little attention has been
hitherto directed to this matter, and we therefore possess little or no
accurate information as to the duration of life in the sexes, but in
some cases we can draw inferences either from anatomical structure or
from the mode of development. Thus, male _Rotifers_ never possess
mouth, stomach, or intestine, they cannot take food, and without doubt
live much shorter lives than the females, which are provided with a
complete alimentary canal. Again, the dwarf males of many parasitic
_Copepods_—low Crustacea—and the ‘complementary males’ of _Cirrhipedes_
(or barnacles) are devoid of stomach, and must live for a much shorter
time than the females; and the male _Entoniscidae_ (a family of which
the species are endo-parasitic in the larger Crustacea), although they
can feed, die after fertilizing the females; while the latter then take
to a parasitic life, produce eggs, and continue to live for some time.
It is supposed that the dwarf male of _Bonellia viridis_ does not live
so long by several years as the hundred times larger female, and it too
has no mouth to its alimentary canal. These examples might be further
increased by reference to zoological literature.

In most cases the female lives longer than the male, and this needs no
special explanation; but the converse relation is conceivable, when,
for instance, the females are much rarer than the males, and the latter
lose much time in seeking them. The above-mentioned case of _Aglia tau_
probably belongs to this category.

We cannot always decide conclusively whether the life of one sex has
been lengthened or that of the other shortened; both these changes must
have taken place in different cases. There is no doubt that a
lengthening of life in the female has arisen in the bees and ants, for
both sexes of the saw-flies, which are believed to be the ancestors of
bees, only live for a few weeks. But among the _Strepsiptera_ the
shorter life of the male must have been secondarily acquired, since we
only rarely meet with such an extreme case in insects.

                             Note 7. Bees.

It has not been experimentally determined whether the workers, which
are usually killed after some months, would live as long as the queen,
if they were artificially protected from danger in the hive; but I
think that this is probable, because it is the case among ants, and
because the peculiarity of longevity must be latent in the egg. As is
well known, the egg which gives rise to the queen is identical with
that which produces a worker, and differences in the nutrition alone
decide whether a queen or a worker shall be formed. It is therefore
probable that the duration of life in queen and worker is potentially
the same.

            Note 8. Death of the Cells in higher Organisms.

The opinion has been often expressed that the inevitable appearance of
normal ‘death’ is dependent on the wearing out of the tissues in
consequence of their functional activity. Bertin says, referring to
animal life[25]:—‘L’observation des faits y attache l’idée d’une
terminaison fatale, bien que la raison ne découvre nullement les motifs
de cette nécessité. Chez les êtres qui font partie du règne animal
l’exercise même de la rénovation moléculaire finit par user le principe
qui l’entretient sans doute parceque le travail d’échange ne
s’accomplissant pas avec une perfection mathématique, il s’établit dans
la figure, comme dans la substance de l’être vivant, une déviation
insensible, et que l’accumulation des écarts finit par amener un type
chimique ou morphologique incompatible avec la persistance de ce
travail.’

Here the replacement of the used-up elements of tissue by new ones is
not taken into account, but an attempt is made to show that the
functions of the whole organism necessarily cause it to waste away. But
the question at once arises, whether such a result does not depend upon
the fact that the single histological elements,—the cells,—are worn out
by the exercise of function. Bertin admits this to be the case, and
this idea of the importance of changes in the cells themselves is
everywhere gaining ground. But although we must admit that the
histological elements do, as a matter of fact, wear out, in
multicellular animals, this would not prove that, nor explain why, such
changes must follow from the nature of the cell and the vital processes
which take place within it. Such an admission would merely suggest the
question:—how is it that the cells in the tissues of higher animals are
worn out by their function, while cells which exist in the form of free
and independent organisms possess the power of living for ever? Why
should not the cells of any tissue, of which the equilibrium is
momentarily disturbed by metabolism, be again restored, so that the
same cells continue to perform their functions for ever:—why cannot
they live without their properties suffering alteration? I have not
sufficiently touched upon this point in the text, and as it is
obviously important it demands further consideration.

In the first place, I think we may conclude with certainty from the
unending duration of unicellular organisms, that such wearing out of
tissue cells is a secondary adaptation, that the death of the cell,
like general death, has arisen with the complex, higher organisms.
Waste does not depend upon the intrinsic nature of the cells, as the
primitive organisms prove to us, but it has appeared as an adaptation
of the cells to the new conditions by which they are surrounded when
they come into combination, and thus form the cell-republic of the
metazoan body. The replacement of cells in the tissues must be more
advantageous for the functions of the whole organism than the unlimited
activity of the same cells, inasmuch as the power of single cells would
be much increased by this means. In certain cases, these advantages are
obvious, as for example in many glands of which the secretions are made
up of cast-off cells. Such cells must die and be separated from the
organism, or the secretion would come to an end. In many cases,
however, the facts are obscure, and await physiological investigation.
But in the meantime we may draw some conclusions from the effects of
growth, which are necessarily bound up with a certain rate of
production of new cells. In the process of growth a certain degree of
choice between the old cells which have performed their functions up to
any particular time, and the new ones which have appeared between them,
is as it were left to the organism.

The organism may thus, figuratively speaking, venture to demand from
the various specific cells of tissues a greater amount of work than
they are able to bear, during the normal length of their life, and with
the normal amount of their strength. The advantages gained by the whole
organism might more than compensate for the disadvantages which follow
from the disappearance of single cells. The glandular secretions which
are composed of cell-detritus, prove that the cells of a complex
organism may acquire functions which result in the loosening of their
connexion with the living cell-community of the body, and their final
separation from it. And the same facts hold with the blood corpuscles,
for the exercise of their function results in ultimate dissolution.
Hence it is not only conceivable, but in every way probable, that many
other functions in the higher organisms involve the death of the cells
which perform them, not because the living cell is necessarily worn out
and finally killed by the exercise of any ordinary vital process, but
because the specific functions in the economy of the cell community
which such cells undertake to perform, involve the death of the cells
themselves. But the fact that such functions have appeared,—involving
as they do the sacrifice of a great number of cells,—entirely depends
upon the replacement of the old by newly formed cells, that is by the
process of reproduction in cells[26].

We cannot _a priori_ dispute the possibility of the existence of
tissues in which the cells are not worn out by the performance of
function, but such an occurrence appears to be improbable when we
recollect that the cells of all tissues owe their constitution to a
very far-reaching process of division of labour, which leaves them
comparatively one-sided, and involves the loss of many properties of
the unicellular, self-sufficient organism. At any rate we only know of
potential immortality in the cells which constitute independent
unicellular organisms, and the nature of these is such that they are
continually undergoing a complete process of reformation.

If we did not find any replacement of cells in the higher organism, we
should be induced to look upon death itself as the direct result of the
division of labour among the cells, and to conclude that the specific
cells of tissues have lost, as a consequence of the one-sided
development of their activities, the power of unending life, which
belongs to all independent primitive cells. We should argue that they
could only perform their functions for a certain time, and would then
die, and with them the organism whose life is dependent upon their
activity. The longer they are occupied with the performance of special
functions, the less completely do they carry out the phenomena of life,
and hence they lead to the appearance of retrogressive changes. But the
replacement of cells is certain in many tissues (in glands, blood,
etc.), so that we can never seek a satisfactory explanation in the
train of reasoning indicated above, but we must assume the existence of
limits to the replacement of cells. In my opinion, we can find an
explanation of this in the general relations of the single individual
to its species, and to the whole of the external conditions of life;
and this is the explanation which I have suggested and have attempted
to work out in the text.

                     Note 9. Death by Sudden Shock.

The most remarkable example of this kind of death known to me, is that
of the male bees. It has been long known that the drone perishes while
pairing, and it was usually believed that the queen bites it to death.
Later observations have however shown that this is not the case, but
that the male suddenly dies during copulation, and that the queen
afterwards bites through the male intromittent organ, in order to free
herself from the dead body. In this case death is obviously due to
sudden excitement, for when the latter is artificially induced, death
immediately follows. Von Berlepsch made some very interesting
observations on this point, ‘If one catches a drone by the wings,
during the nuptial flight, and holds it free in the air without
touching any other part, the penis is protruded and the animal
instantly dies, becoming motionless as though killed by a shock. The
same thing happens if one gently stimulates the dorsal surface of the
drone on a similar occasion. The male is in such an excited and
irritable condition that the slightest muscular movement or disturbance
causes the penis to be protruded[27].’ In this case death is caused by
the so-called nervous shock. The humble-bees are not similarly
constituted, for the male does not die after fertilizing the female,
‘but withdraws its penis and flies away.’ But the death of male bees,
during pairing, must not be regarded as normal death. Experiment has
shown that these insects can live for more than four months[28]. They
do not, as a matter of fact, generally live so long; for—although the
workers do not, as was formerly believed, kill them after the
fertilization of the queen, by direct means—they prevent them from
eating the honey and drive them from the hive, so that they die of
hunger[29].

We must also look upon death which immediately, or very quickly,
follows upon the deposition of eggs as death by sudden shock. The
females of certain species of _Psychidae_, when they reproduce
sexually, may remain alive for more than a week waiting for a male:
after fertilization, however, they lay their eggs and die, while the
parthenogenetic females of the same species lay their eggs and die
immediately after leaving the cocoon; so that while the former live for
many days, the latter do not last for more than twenty-four hours. ‘The
parthenogenetic form of _Solenobia triquetrella_, soon after emergence,
lays all her eggs together in the empty case, becomes much shrunken,
and dies in a few hours.’ (Letter from Dr. Speyer, Rhoden.)

Note 10. Intermingling during the Fission of Unicellular Organisms[30].

Fission is quite symmetrical in _Amoebae_, so that it is impossible to
recognise mother and daughter in the two resulting organisms. But in
_Euglypha_ and allied forms the existence of a shell introduces a
distinguishing mark by which it is possible to discriminate between the
products of fission; so that the offspring can be differentiated from
the parent. The parent organism, before division, builds the parts of
the shell for the daughter form. These parts are arranged on the
surface of that part of the protoplasm, external to the old shell,
which will be subsequently separated as the daughter-cell. On this part
the spicules are arranged and unite to form the new shell. The division
of the nucleus takes place after that of the protoplasm, so that the
daughter-cell is for some time without a nucleus. Although we can in
this species recognise the daughter-cell for some time after separation
from the parent by the greater transparency of its younger shell, it is
nevertheless impossible to admit that the characteristics of the two
animals are in any way different, for just before the separation of the
two individuals a circulation of the protoplasm through both shells
takes place after the manner described in the text, and there is
therefore a complete intermingling of the substance of the two bodies.

The difference between the products is even greater after transverse
fission of the _Infusoria_, for a new anus must be formed at the
anterior part and a new mouth posteriorly. It is not known whether any
circulation of the protoplasm takes place, as in _Euglypha_. But even
if this does not occur, there is no reason for believing that the two
products of division possess a different duration of life.

The process of fission in the _Diatomaceae_ seems to me to be
theoretically important, because here, as in the previously-mentioned
_Monothalamia_ (_Euglypha_, etc.), the new silicious skeleton is built
up within the primary organism, but not, as in _Euglypha_, for the new
individual only, but for both parent and daughter-cell alike[31]. If we
compare the diatom shell to a box, then the two halves of the old shell
would form two lids, one for each of the products of fission, while a
new box is built up afresh for each of them. In this case there is an
absolute equality between the products of fission, so far as the shell
is concerned.

                         Note 11. Regeneration.

A number of experiments have been recently undertaken, in connection
with a prize thesis at Würzburg, in order to test the powers of
regeneration possessed by various animals. In all essential respects
the results confirm the statements of the older observers, such as
Spallanzani. Carrière has also proved that snails can regenerate not
only their horns and eyes, but also part of the head when it has been
cut off, although he has shown that Spallanzani's old statement that
they can regenerate the whole head, including the nervous system, is
erroneous[32].

                Note 12. The Duration of Life in Plants.

The title of the work on this subject mentioned in the Text is ‘Die
Lebensdauer und Vegetationsweise der Pflanzen, ihre Ursache und ihre
Entwicklung,’ F. Hildebrand, Engler’s botanische Jahrbücher, Bd. II. 1.
und 2. Heft, Leipzig, 1881.

                                Note 13.

[Many interesting facts and conclusions upon the subject of this essay
will be found in a volume by Professor E. Ray Lankester, ‘On
comparative Longevity in Man and the lower Animals,’ Macmillan and Co.,
1870.—E. B. P.]

------------------------------------------------------------------------

                 Footnotes for the Appendix to Essay I.

Footnote 1:

  Humboldt’s ‘Ausichten der Natur.’

Footnote 2:

  This estimate is derived from observation of the time during which
  these insects are to be seen upon the wing. Direct observations upon
  the duration of life in this species are unknown to me.

Footnote 3:

  [Sir John Lubbock has now kept a queen ant alive for nearly 15 years.
  See note 2 {note 18 below} on p. 51.—E. B. P.]

Footnote 4:

  [After reading these proofs Dr. A. R. Wallace kindly sent me an
  unpublished note upon the production of death by means of natural
  selection, written by him some time between 1865 and 1870. The note
  contains some ideas on the subject, which were jotted down for
  further elaboration, and were then forgotten until recalled by the
  argument of this Essay. The note is of great interest in relation to
  Dr. Weismann’s suggestions, and with Dr. Wallace’s permission I print
  it in full below.

  ‘The Action of Natural Selection in Producing Old Age, Decay, and
  Death.

  ‘Supposing organisms ever existed that had not the power of natural
  reproduction, then since the absorptive surface would only increase
  as the square of the dimensions while the bulk to be nourished and
  renewed would increase as the cube, there must soon arrive a limit
  of growth. Now if such an organism did not produce its like,
  accidental destruction would put an end to the species. Any
  organism therefore that, by accidental or spontaneous fission,
  could become two organisms, and thus multiply itself indefinitely
  without increasing in size beyond the limits most favourable for
  nourishment and existence, could not be thus exterminated: since
  the individual only could be accidentally destroyed,—the race would
  survive. But if individuals did not die they would soon multiply
  inordinately and would interfere with each other’s healthy
  existence. Food would become scarce, and hence the larger
  individuals would probably decompose or diminish in size. The
  deficiency of nourishment would lead to parts of the organism not
  being renewed; they would become fixed, and liable to more or less
  slow decomposition as dead parts within a living body. The smaller
  organisms would have a better chance of finding food, the larger
  ones less chance. That one which gave off several small portions to
  form each a new organism would have a better chance of leaving
  descendants like itself than one which divided equally or gave off
  a large part of itself. Hence it would happen that those which gave
  off very small portions would probably soon after cease to maintain
  their own existence while they would leave a numerous offspring.
  This state of things would be in any case for the advantage of the
  race, and would therefore, by natural selection, soon become
  established as the regular course of things, and thus we have the
  origin of _old age_, _decay_, and _death_; for it is evident that
  when one or more individuals have provided a sufficient number of
  successors they themselves, as consumers of nourishment in a
  constantly increasing degree, are an injury to those successors.
  Natural selection therefore weeds them out, and in many cases
  favours such races as die almost immediately after they have left
  successors. Many moths and other insects are in this condition,
  living only to propagate their kind and then immediately dying,
  some not even taking any food in the perfect and reproductive
  state.’—E. B. P.]

Footnote 5:

  Johannes Müller, ‘Physiologie,’ Bd. I. p. 31, Berlin, 1840.

Footnote 6:

  Oken, ‘Naturgeschichte,’ Stuttgart, 1837, Bd. IV. Abth. 1.

Footnote 7:

  Brehm, ‘Leben der Vögel,’ p. 278.

Footnote 8:

  ‘Naturwissenschaftliche Thatsachen und Probleme,’ Populäre Vorträge,
  Berlin, 1880; _vide_ Appendix.

Footnote 9:

  ‘Entomolog. Mag.,’ vol. i. p. 527, 1833.

Footnote 10:

  Imhof, ‘Beiträge zur Anatomie der _Perla maxima_,’ Inaug. Diss.,
  Aarau, 1881.

Footnote 11:

  Mr. Edwards has meanwhile published these communications in full; cf.
  ‘On the length of life of Butterflies,’ Canadian Entomologist, 1881,
  p. 205.

Footnote 12:

  When no authority is given, the observations are my own.

Footnote 13:

  In the paper quoted above, Edwards, after weighing all the evidence,
  reduces the length of life from three to four weeks.

Footnote 14:

  ‘Entomolog. Mag.,’ vol. i. p. 527, 1823.

Footnote 15:

  Ibid.

Footnote 16:

  Ibid.

Footnote 17:

  ‘Recherches sur les mœurs des Fourmis indigènes,’ Genève, 1810.

Footnote 18:

  These two female ants were still alive on the 25th of September
  following Sir John Lubbock’s letter, so that they live at least seven
  years. Cf. ‘Observations on Ants, Bees, and Wasps,’ Part VIII. p.
  385; Linn. Soc. Journ. Zool., vol. xv. 1881.

  [Sir John Lubbock has kindly given me further information upon the
  duration of life of these two queen ants. Since the receipt of his
  letter, the facts have been published in the Journal of the Linnean
  Society (Zoology), vol. xx. p. 133. I quote in full the passage which
  refers to these ants:—

  ‘Longevity.—It may be remembered that my nests have enabled me to
  keep ants under observation for long periods, and that I have
  identified workers of _Lasius niger_ and _Formica fusca_ which were
  at least seven years old, and two queens of _Formica fusca_ which
  have lived with me ever since December 1874. One of these queens,
  after ailing for some days, died on the 30th July, 1887. She must
  then have been more than thirteen years old. I was at first afraid
  that the other one might be affected by the death of her companion.
  She lived, however, until the 8th August, 1888, when she must have
  been nearly fifteen years old, and is therefore by far the oldest
  insect on record.

  ‘Moreover, what is very extraordinary, she continued to lay fertile
  eggs. This remarkable fact is most interesting from a physiological
  point of view. Fertilization took place in 1874 at the latest. There
  has been no male in the nest since then, and, moreover, it is, I
  believe, well established that queen ants and queen bees are
  fertilized once for all. Hence the spermatozoa of 1874 must have
  retained their life and energy for thirteen years, a fact, I believe,
  unparalleled in physiology.’

                  *       *       *       *       *

  ‘I had another queen of _Formica fusca_ which lived to be thirteen
  years old, and I have now a queen of _Lasius niger_ which is more
  than nine years old, and still lays fertile eggs, which produce
  female ants.’

  Both the above-mentioned queens may have been considerably older, for
  it is impossible to estimate their age at the time of capture. It is
  only certain (as Sir John Lubbock informs me in his letter) that they
  must have been at least nine months old (when captured), as the eggs
  of _F. fusca_ are laid in March or early in April.’ The queens became
  gradually ‘somewhat lethargic and stiff in their movements (before
  their death), but there was no loss of any limb nor any abrasion.’
  This last observation seems to indicate that queen ants may live for
  a much longer period in the wild state, for it is stated above that
  the chitin is often greatly worn, and some of the limbs lost (see pp.
  48, 51, and 52).—E. B. P.]

Footnote 19:

  A. von Berlepsch, ‘Die Biene und ihre Zucht,’ etc., 3rd ed.;
  Mannheim, 1872.

Footnote 20:

  E. Bevan, ‘Ueber die Honigbiene und die Länge ihres Lebens;’ abstract
  in Oken’s ‘Isis,’ 1844, p. 506.

Footnote 21:

  Dalyell, ‘Rare and Remarkable Animals of Scotland,’ vol. ii. p. 203;
  London, 1848.

Footnote 22:

  [Mr. J. S. Haldane has kindly obtained details of the death of the
  sea anemone referred to by the author. It died, by a natural death,
  on August 4, 1887, after having appeared to become gradually weaker
  for some months previous to this date. It had lived ever since 1828
  in the same small glass jar in which it was placed by Sir John
  Dalyell. It must have been at least 66 years old when it died.—E.B.P.]

Footnote 23:

  Bronn, ‘Klassen und Ordnungen des Thierreichs,’ Bd. III. p. 466;
  Leipzig.

Footnote 24:

  Bronn, l. c.

Footnote 25:

  Cf. the article ‘Mort’ in the ‘Encyclop. Scienc. Méd.’ vol. M. p. 520.

Footnote 26:

  Roux, in his work ‘Der Kampf der Theile im Organismus,’ Jena 1881,
  has attempted to explain the manner in which division of labour has
  arisen among the cells of the higher organisms, and to render
  intelligible the mechanical processes by which the purposeful
  adaptations of the organism have arisen.

Footnote 27:

  von Berlepsch, ‘Die Biene und ihre Zucht,’ etc.

Footnote 28:

  Oken, ‘Isis,’ 1844, p. 506.

Footnote 29:

  von Berlepsch, l. c., p. 165.

Footnote 30:

   Cf. August Gruber, ‘Der Theilungsvorgang bei Euglypha alveolata,’
  and ‘Die Theilung der monothalamen Rhizopoden,’ Z. f. W. Z., Bd.
  XXXV. and XXXVI., p. 104, 1881.

Footnote 31:

  Cf. Victor Hensen, ‘Physiologie d. Zeugung,’ p. 152.

Footnote 32:

  Cf. J. Carrière, ‘Ueber Regeneration bei Landpulmonaten,’ Tagebl. der
  52. Versammlg. deutsch. Naturf. pp. 225-226.

------------------------------------------------------------------------






                                  II.

                              ON HEREDITY.

                                 1883.




------------------------------------------------------------------------

                              ON HEREDITY.

                                PREFACE.

The following essay was my inaugural lecture as Pro-Rector of the
University of Freiburg, and was delivered publicly in the hall of the
University, on June 21, 1883; it first appeared in print in the
following August. Only a few copies of the first edition were available
for the public, and it is therefore now reprinted as a second edition,
which only differs from the first in a few not unimportant improvements
and additions.

The title which I have chosen requires some explanation. I do not
propose to treat of the whole problem of heredity, but only of a
certain aspect of it—the transmission of acquired characters which has
been hitherto assumed to occur. In taking this course I may say that it
was impossible to avoid going back to the foundation of all the
phenomena of heredity, and to determine the substance with which they
must be connected. In my opinion this can only be the substance of the
germ-cells; and this substance transfers its hereditary tendencies from
generation to generation, at first unchanged, and always uninfluenced
in any corresponding manner, by that which happens during the life of
the individual which bears it. If these views, which are indicated
rather than elaborated in this paper, be correct, all our ideas upon
the transformation of species require thorough modification, for the
whole principle of evolution by means of exercise (use and disuse), as
proposed by Lamarck, and accepted in some cases by Darwin, entirely
collapses.

The nature of the present paper—which is a lecture and not an elaborate
treatise—necessitates that only suggestions and not an exhaustive
treatment of the subject could be given. I have also abstained from
giving further details in the form of an appendix, chiefly because I
could hardly have attempted to complete a treatment of the whole range
of the subject, and I hope to refer again to these questions in the
future, when new experiments and observations have been made.

I am very glad to see that such an important authority as Pflüger[33]
has in the meantime come to the same opinion, from an entirely
different direction—an opinion which forms the foundation of the views
here brought forward, namely, that heredity depends upon the continuity
of the molecular substance of the germ from generation to generation.

                                                               A. W.

                                  II.

                              ON HEREDITY.

With your permission I wish to bring before you to-day my views on a
problem of general biological interest—the problem of heredity.

Heredity is the process which renders possible that persistence of
organic beings throughout successive generations, which is generally
thought to be so well understood and to need no special explanation.
Nevertheless our minds cannot fail to be much perplexed by the
multiplicity of its manifestations, and to be greatly puzzled as to its
real nature. A celebrated German physiologist says[34], ‘Although many
hands have at all times endeavoured to break the seal which hides the
theory of heredity from our view, the results achieved have been but
small; and we are in a certain degree justified in looking with little
hope upon new efforts undertaken in this direction. We must
nevertheless endeavour from time to time to ascertain how far we have
advanced towards a complete explanation.’

Such a course is in every way advisable, for we are not dealing with
phenomena which from their very nature are incomprehensible by man. The
great complexity of the subject has alone rendered it hitherto
insuperable, but in the province of heredity we certainly have not
reached the limits of attainable knowledge.

From this point of view heredity bears some resemblance to certain
anatomical and physiological problems, e. g. the structure and function
of the human brain. Its structure—with so many millions of nerve-fibres
and nerve-cells—is of such extraordinary complexity that we might well
despair of ever completely understanding it. Each fibre is nevertheless
distinct in itself, while its connection with the nearest nerve-cell
can be frequently traced, and the function of many groups of cell
elements is already known. But it would seem to be impossible to
unravel the excessively complex network into which the cells and fibres
are knit together; and hence to arrive at the function of each single
element appears to be also beyond our reach. We have not however
commenced to untie this Gordian knot without some hope of success, for
who can say how far human perseverance may be able to penetrate into
the mechanism of the brain, and to reveal a connected structure and a
common principle in its countless elements? But surely this work will
be most materially assisted by the simultaneous investigation of the
structure and function of the nervous system in the lower forms of
life—in the polypes and jelly-fish, worms and Crustacea. In the same
way we should not abandon the hope of arriving at a satisfactory
knowledge of the processes of heredity, if we consider the simplest
processes of the lower animals as well as the more complex processes
met with in the higher forms.

The word heredity in its common acceptation, means that property of an
organism by which its peculiar nature is transmitted to its
descendants. From an eagle’s egg an eagle of the same species
developes; and not only are the characteristics of the species
transmitted to the following generation, but even the individual
peculiarities. The offspring resemble their parents among animals as
well as among men.

On what does this common property of all organisms depend?

Häckel was probably the first to describe reproduction as ‘an
overgrowth of the individual,’ and he attempted to explain heredity as
a simple continuity of growth. This definition might be considered as a
play upon words, but it is more than this; and such an interpretation
rightly applied, points to the only path which, in my opinion, can lead
to the comprehension of heredity.

Unicellular organisms, such as Rhizopoda and Infusoria, increase by
means of fission. Each individual grows to a certain size, and then
divides into two parts, which are exactly alike in size and structure,
so that it is impossible to decide whether one of them is younger or
older than the other. Hence in a certain sense these organisms possess
immortality: they can, it is true, be destroyed, but, if protected from
a violent death, they would live on indefinitely, and would only from
time to time reduce the size of their overgrown bodies by division.
Each individual of any such unicellular species living on the earth
to-day is far older than mankind, and is almost as old as life itself.

From these unicellular organisms we can to a certain extent understand
why the offspring, being in fact a part of its parents, must therefore
resemble the latter. The question as to why the part should resemble
the whole leads us to a new problem, that of assimilation, which also
awaits solution. It is, at any rate, an undoubted fact that the
organism possesses the power of taking up certain foreign substances,
viz. food, and of converting them into the substance of its own body.

Among these unicellular organisms, heredity depends upon the continuity
of the individual during the continual increase of its body by means of
assimilation.

But how is it with the multicellular organisms which do not reproduce
by means of simple division, and in which the whole body of the parent
does not pass over into the offspring?

In such animals sexual reproduction is the chief means of
multiplication. In no case has it always been completely wanting, and
in the majority of cases it is the only kind of reproduction.

In these animals the power of reproduction is connected with certain
cells which, as germ-cells, may be contrasted with those which form the
rest of the body; for the former have a totally different rôle to play;
they are without significance for the life of the individual[35], and
yet they alone possess the power of preserving the species. Each of
them can, under certain conditions, develope into a complete organism
of the same species as the parent, with every individual peculiarity of
the latter reproduced more or less completely. How can such hereditary
transmission of the characters of the parent take place? how can a
single reproductive cell reproduce the whole body in all its details?

Such a question could be easily answered if we were only concerned with
the continuity of the substance of the reproductive cells from one
generation to another; for this can be demonstrated in some cases, and
is very probable in all. In certain insects the development of the egg
into the embryo, that is the segmentation of the egg, begins with the
separation of a few small cells from the main body of the egg. These
are the reproductive cells, and at a later period they are taken into
the interior of the animal and form its reproductive organs. Again, in
certain small freshwater Crustacea (_Daphnidae_) the future
reproductive cells become distinct at a very early period, although not
quite at the beginning of segmentation, i. e. when the egg has divided
into not more than thirty segments. Here also the cells which are
separated early form the reproductive organs of the animal. The
separation of the reproductive cells from those of the body takes place
at a still later period, viz. at the close of segmentation, in
_Sagitta_—a pelagic free-swimming form. In Vertebrata they do not
become distinct from the other cells of the body until the embryo is
completely formed. Thus, as their development shows, a marked
antithesis exists between the substance of the undying reproductive
cells and that of the perishable body-cells. We cannot explain this
fact except by the supposition that each reproductive cell potentially
contains two kinds of substance, which at a variable time after the
commencement of embryonic development, separate from one another, and
finally produce two sharply contrasted groups of cells.

It is evidently unimportant, as regards the question of heredity,
whether this separation takes place early or late, inasmuch as the
molecular constitution of the reproductive substance is determined
before the beginning of development. In order to understand the growth
and multiplication of cells, it must be conceded that all protoplasmic
molecules possess the power of growing, that is of assimilating food,
and of increasing by means of division. In the same manner the
molecules of the reproductive protoplasm, when well nourished, grow and
increase without altering their peculiar nature, and without modifying
the hereditary tendencies derived from the parents. It is therefore
quite conceivable that the reproductive cells might separate from the
somatic cells much later than in the examples mentioned above, without
changing the hereditary tendencies of which they are the bearers. There
may be in fact cases in which such separation does not take place until
after the animal is completely formed, and others, as I believe that I
have shown[36], in which it first arises one or more generations later,
viz. in the buds produced by the parent. Here also there is no ground
for the belief that the hereditary tendencies of the reproductive
molecules are in any way changed by the length of time which elapses
before their separation from the somatic molecules. And this
theoretical deduction is confirmed by observation, for from the egg of
a Medusa, produced by the budding of a Polype, a Polype, in the first
instance, and not a Medusa arises. Here the molecules of the
reproductive substance first formed part of the Polype, and later, part
of the Medusa bud, and, although they separated from the somatic cells
in the bud, they nevertheless always retain the tendency to develope
into a Polype.

We thus find that the reproduction of multicellular organisms is
essentially similar to the corresponding process in unicellular forms;
for it consists in the continual division of the reproductive cell; the
only difference being that in the former case the reproductive cell
does not form the whole individual, for the latter is composed of the
millions of somatic cells by which the reproductive cell is surrounded.
The question, ‘How can a single reproductive cell contain the germ of a
complete and highly complex individual?’ must therefore be re-stated
more precisely in the following form, ‘How can the substance of the
reproductive cells potentially contain the somatic substance with all
its characteristic properties?’

The problem which this question suggests, becomes clearer when we
employ it for the explanation of a definite instance, such as the
origin of multicellular from unicellular animals. There can be no doubt
that the former have originated from the latter, and that the
physiological principle upon which such an origin depended, is the
principle of division of labour. In the course of the phyletic
development of the organized world, it must have happened that certain
unicellular individuals did not separate from one another immediately
after division, but lived together, at first as equivalent elements,
each of which retained all the animal functions, including that of
reproduction. The _Magosphaera planula_ of Häckel proves that such
perfectly homogeneous cell-colonies exist[37], even at the present day.
Division of labour would produce a differentiation of the single cells
in such a colony: thus certain cells would be set apart for obtaining
food and for locomotion, while certain other cells would be exclusively
reproductive. In this way colonies consisting of somatic and of
reproductive cells must have arisen, and among these for the first time
death appeared. For in each case the somatic cells must have perished
after a certain time, while the reproductive cells alone retained the
immortality inherited from the Protozoa. We must now ask how it becomes
possible that one kind of cell in such a colony, can produce the other
kind by division? Before the differentiation of the colony each cell
always produced others similar to itself. How can the cells, after the
nature of one part of the colony is changed, have undergone such
changes in _their_ nature that they can now produce more than one kind
of cell?

Two theories can be brought forward to solve this problem. We may turn
to the old and long since abandoned _nisus formativus_, or adapting the
name to modern times, to a phyletic force of development which causes
the organism to change from time to time. This _vis a tergo_ or
teleological force compels the organism to undergo new transformations
without any reference to the external conditions of life. This theory
throws no light upon the numerous adaptations which are met with in
every organism; and it possesses no value as a scientific explanation.

Another supposition is that the primary reproductive cells are
influenced by the secondary cells of the colony, which, by their
adaptability to the external conditions of life, have become somatic
cells: that the latter give off minute particles which entering into
the former, cause such changes in their nature that at the next
succeeding cell-division they are compelled to break up into dissimilar
parts.

At first sight this hypothesis seems to be quite reasonable. It is not
only conceivable that particles might proceed from the somatic to the
reproductive cells, but the very nutrition of the latter at the expense
of the former is a demonstration that such a passage actually takes
place. But a closer examination reveals immense difficulties. In the
first place, the molecules of the body devoured are never simply added
to those of the feeding individual without undergoing any change, but
as far as we know, they are really assimilated[38], that is, converted
into the molecules of the latter. We cannot therefore gain much by
assuming that a number of molecules can pass from the growing somatic
cells into the growing reproductive cells, and can be deposited
unchanged in the latter, so that, at their next division, the molecules
are separated to become the somatic cells of the following generation.
How can such a process be conceivable, when the colony becomes more
complex, when the number of somatic cells becomes so large that they
surround the reproductive cells with many layers, and when at the same
time by an increasing division of labour a great number of different
tissues and cells are produced, all of which must originate _de novo_
from a single reproductive cell? Each of these various elements must,
_ex hypothesi_, give up certain molecules to the reproductive cells;
hence those which are in immediate contact with the latter would
obviously possess an advantage over those which are more remote. If
then any somatic cell must send the same number of molecules to each
reproductive cell[39], we are compelled to suspend all known physical
and physiological conceptions, and must make the entirely gratuitous
assumption of an affinity on the part of the molecules for the
reproductive cells. Even if we admit the existence of this affinity,
its origin and means of control remain perfectly unintelligible if we
suppose that it has arisen from differentiation of the complete colony.
An unknown controlling force must be added to this mysterious
arrangement, in order to marshal the molecules which enter the
reproductive cell in such a manner that their arrangement corresponds
with the order in which they must emerge as cells at a later period. In
short, we become lost in unfounded hypotheses.

It is well known that Darwin has attempted to explain the phenomena of
heredity by means of a hypothesis which corresponds to a considerable
extent with that just described. If we substitute gemmules for
molecules we have the fundamental idea of Darwin’s provisional
hypothesis of pangenesis. Particles of an excessively minute size are
continually given off from all the cells of the body; these particles
collect in the reproductive cells, and hence any change arising in the
organism, at any time during its life, is represented in the
reproductive cell[40]. Darwin believed that he had by this means
rendered the transmission of acquired characters intelligible, a
conception which he held to be necessary in order to explain the
development of species. He himself pointed out that the hypothesis was
merely provisional, and that it was only an expression of immediate,
and by no means satisfactory knowledge of these phenomena.

It is always dangerous to invoke some entirely new force in order to
understand phenomena which cannot be readily explained by the forces
which are already known.

I believe that an explanation can in this case be reached by an appeal
to known forces, if we suppose that characters acquired (in the true
sense of the term) by the parent cannot appear in the course of the
development of the offspring, but that all the characters exhibited by
the latter are due to primary changes in the germ.

This supposition can obviously be made with regard to the
above-mentioned colony with its constituent elements differentiated
into somatic and reproductive cells. It is conceivable that the
differentiation of the somatic cells was not primarily caused by a
change in their own structure, but that it was prepared for by changes
in the molecular structure of the reproductive cell from which the
colony arose.

The generally received idea assumes that changes in the external
conditions can, in connection with natural selection, call forth
persistent changes in an organism; and if this view be accepted it must
be as true of all Metazoa as it is of unicellular or of homogeneous
multicellular organisms. Supposing that the hypothetical colonies,
which were at first entirely made up of similar cells, were to gain
some advantages, if in the course of development, the molecules of the
reproductive cells, from which each colony arose became distributed
irregularly in the resulting organism, there would be a tendency
towards the perpetuation of such a change, wherever it appeared as the
result of individual variability. As a result of this change the colony
would no longer remain homogeneous, and its cells would become
dissimilar from the first, because of the altered arrangement of the
molecules in the reproductive cells. Nothing prevents us from assuming
that, at the same time, the nature of a part of the molecule may
undergo still further change, for the molecules are by nature complex,
and may split up or combine together.

If then the reproductive cells have undergone such changes that they
can produce a heterogeneous colony as the result of continual division,
it follows that succeeding generations must behave in exactly the same
manner, for each of them is developed from a portion of the
reproductive cell from which the previous generation arose, and
consists of the same reproductive substance as the latter.

From this point of view the exact manner in which we imagine the
subsequent differentiation of the colony to be potentially present in
the reproductive cell, becomes a matter of comparatively small
importance. It may consist in a different molecular arrangement, or in
some change of chemical constitution, or it may be due to both these
causes combined. The essential point is that the differentiation was
originally due to some change in the reproductive cells, just as this
change itself produces all the differentiations which appear in the
ontogeny of all species at the present day. No one doubts that the
reason why this or that form of segmentation takes place, or why this
or that species finally appears, is to be found in the ultimate
structure of the reproductive cells. And, as a matter of fact,
molecular differentiation and grouping, whether present from the
beginning or first appearing in the course of development, plays a rôle
which can be almost directly observed in certain species. The first
segmentation furrow divides the egg of such species into an opaque and
a clear half, or, as is often the case among Medusae, into a granular
outer layer and a clear central part, corresponding respectively with
the ectoderm and endoderm which are formed at a later period. Such
early differentiations are only the visible proofs of certain highly
complex molecular rearrangements in the cells, and the fact appears to
indicate that we cannot be far wrong in maintaining that
differentiations which appear in the course of ontogeny depend upon the
chemical and physical constitution of the molecules in the reproductive
cell.

At the first appearance of the earliest Metazoa alluded to above, only
two kinds of cells, somatic and reproductive, arose from the
segmentation of the reproductive cell. The reproductive cells thus
formed must have possessed exactly the same molecular structure as the
mother reproductive cell, and would therefore pass through precisely
the same developmental changes. We can easily imagine that all the
succeeding stages in the development of the Metazoa have been due to
the same causes which were efficient at the earliest period. Variations
in the molecular structure of the reproductive cells would continue to
appear, and these would be increased and rendered permanent by means of
natural selection, when their results, in the alteration of certain
cells in the body, were advantageous to the species. The only condition
necessary for the transmission of such changes is that a part of the
reproductive substance (the germ-plasm) should always remain unchanged
during segmentation and the subsequent building up of the body, or in
other words, that such unchanged substance should pass into the
organism, and after the lapse of a variable period, should reappear as
the reproductive cells. Only in this way can we render to some extent
intelligible the transmission of those changes which have arisen in the
phylogeny of the species; only thus can we imagine the manner in which
the first somatic cells gradually developed in numbers and in
complexity.

It is only by supposing that these changes arose from molecular
alterations in the reproductive cell that we can understand how the
reproductive cells of the next generation can originate the same
changes in the cells which are developed from them; and it is
impossible to imagine any way in which the transmission of changes,
produced by the direct action of external forces upon the somatic
cells, can be brought about[41].

The difficulty or the impossibility of rendering the transmission of
acquired characters intelligible by an appeal to any known force has
been often felt, but no one has hitherto attempted to cast doubts upon
the very existence of such a form of heredity.

There are two reasons for this: first, observations have been recorded
which appear to prove the existence of such transmission; and secondly,
it has seemed impossible to do without the supposition of the
transmission of acquired characters, because it has always played such
an important part in the explanation of the transformation of species.

It is perfectly right to defer an explanation, and to hesitate before
we declare a supposed phenomenon to be impossible, because we are
unable to refer it to any of the known forces. No one can believe that
we are acquainted with all the forces of nature. But, on the other
hand, we must use the greatest caution in dealing with unknown forces;
and clear and indubitable facts must be brought forward to prove that
the supposed phenomena have a real existence, and that their acceptance
is unavoidable.

It has never been proved that acquired characters are transmitted, and
it has never been demonstrated that, without the aid of such
transmission, the evolution of the organic world becomes unintelligible.

The inheritance of acquired characters has never been proved, either by
means of direct observation or by experiment[42]. It must be admitted
that there are in existence numerous descriptions of cases which tend
to prove that such mutilations as the loss of fingers, the scars of
wounds, etc., are inherited by the offspring, but in these descriptions
the previous history is invariably obscure, and hence the evidence
loses all scientific value.

As a typical example of the scientific value of such cases I may
mention the frequently quoted instance of the cow, which lost its left
horn from suppuration, induced by some ‘unknown cause,’ and which
afterwards produced two calves with a rudimentary left horn in each
case. But as Hensen[43] has rightly remarked, the loss of the cow’s
horn may have arisen from a congenital malformation, which would
certainly be transmitted, but which was not an acquired character.

The only cases worthy of scientific discussion are the well-known
experiments upon guinea-pigs, conducted by the French physiologist
Brown-Séquard. But the explanation of his results is, in my opinion,
open to discussion. In these cases we have to do with the apparent
transmission of artificially produced malformations. The division of
important nerves, or of the spinal cord, or the removal of parts of the
brain, produced certain symptoms which reappeared in the descendants of
the mutilated animals. Epilepsy was produced by dividing the great
sciatic nerve; the ear became deformed when the sympathetic nerve was
severed in the throat; and prolapsus of the eye-ball followed the
removal of a certain part of the brain—the corpora restiformia. All
these effects were said to be transmitted to the descendants as far as
the fifth or sixth generation.

But we must inquire whether these cases are really due to heredity and
not to simple infection. In the case of epilepsy, at any rate, it is
easy to imagine that the passage of some specific organism through the
reproductive cells may take place, as in the case of syphilis. We are,
however, entirely ignorant of the nature of the former disease. This
suggested explanation may not perhaps apply to the other cases: but we
must remember that animals which have been subjected to such severe
operations upon the nervous system have sustained a great shock, and if
they are capable of breeding, it is only probable that they will
produce weak descendants, and such as are easily affected by disease.
Such a result does not however explain why the offspring should suffer
from the same disease as that which was artificially induced in the
parents. But this does not appear to have been by any means invariably
the case. Brown-Séquard himself says, ‘The changes in the eye of the
offspring were of a very variable nature, and were only occasionally
exactly similar to those observed in the parents.’

There is no doubt, however, that these experiments demand careful
consideration, but before they can claim scientific recognition, they
must be subjected to rigid criticism as to the precautions taken, the
number and nature of the control experiments, etc.

Up to the present time such necessary conditions have not been
sufficiently observed. The recent experiments themselves are only
described in short preliminary notices, which, as regards their
accuracy, the possibility of mistake, the precautions taken, and the
exact succession of individuals affected, afford no data upon which a
scientific opinion can be founded. Until the publication of a complete
series of experiments, we must say with Du Bois Reymond[44], ‘The
hereditary transmission of acquired characters remains an
unintelligible hypothesis, which is only deduced from the facts which
it attempts to explain.’

We therefore naturally ask whether the hypothesis is really necessary
for the explanation of known facts.

At the first sight it certainly seems to be necessary, and it appears
rash to attempt to dispense with its aid. Many phenomena only appear to
be intelligible if we assume the hereditary transmission of such
acquired characters as the changes which we ascribe to the use or
disuse of particular organs, or to the direct influence of climate.
Furthermore, how can we explain instinct as hereditary habit unless it
has gradually arisen by the accumulation, through heredity, of habits
which were practised in succeeding generations?

I will now attempt to prove that even these cases, so far as they
depend upon clear and indubitable facts, do not force us to accept the
supposition of the transmission of acquired characters.

It seems difficult and well nigh impossible to deny the transmission of
acquired characters when we remember the influence which use and disuse
have exercised upon certain special organs. It is well known that
Lamarck attempted to explain the structure of the organism as almost
entirely due to this principle alone. According to his theory the long
neck of the giraffe arose by constant stretching after the leaves of
trees, and the web between the toes of a water-bird’s foot by the
extension of the toes, in an attempt to oppose as large a surface of
water as possible in swimming. There can be no doubt that those muscles
which are frequently used increase in size and strength, and that
glands which often enter into activity become larger and not smaller,
and that their functional powers increase. Indeed, the whole effect
which exercise produces upon the single parts of the body is dependent
upon the fact that frequently used organs increase in strength. This
conclusion also refers to the nervous system, for a pianist who
performs with lightning rapidity certain pre-arranged, highly complex,
and combined movements of the muscles of his hands and fingers has, as
Du Bois Reymond pointed out, not only exercised the muscles, but also
those ganglionic centres of the brain which determine the combination
of muscular movement. Other functions of the brain, such as memory, can
be similarly increased and strengthened by exercise, and the question
to be settled is whether characters acquired in this way by exercise
and practice can be transmitted to the following generations. Lamarck’s
theory assumes that such transmission takes place, for without it no
accumulation or increase of the characters in question would be
possible, as a result of their exercise during any number of successive
generations.

Against this we may urge that whenever, in the course of nature, an
organ becomes stronger by exercise, it must possess a certain degree of
importance for the life of the individual, and when this is the case it
becomes subject to improvement by natural selection, for only those
individuals which possess the organ in its most perfect form will be
able to reproduce them. The perfection of form of an organ does not
however depend upon the amount of exercise undergone by it during the
life of the organism, but primarily and principally upon the fact that
the germ from which the individual arose was predisposed to produce a
perfect organ. The increase to which any organ can attain by exercise
during a single life is bounded by certain limits, which are themselves
fixed by the primary tendencies of the organ in question. We cannot by
excessive feeding make a giant out of the germ destined to form a
dwarf; we cannot, by means of exercise, transform the muscles of an
individual destined to be feeble into those of a Hercules, or the brain
of a predestined fool into that of a Leibnitz or a Kant, by means of
much thinking. With the same amount of exercise the organ which is
destined to be strong, will attain a higher degree of functional
activity than one that is destined to be weak. Hence natural selection,
in destroying the least fitted individuals, destroys those which from
the germ were feebly disposed. Thus the result of exercise during the
individual life does not acquire so much importance, for, as compared
with differences in predisposition, the amount of exercise undergone by
all the individuals of a species becomes relatively uniform. The
increase of an organ in the course of generations does not depend upon
the summation of the exercise taken during single lives, but upon the
summation of more favourable predispositions in the germs.

In criticizing these arguments, it may be questioned whether the single
individuals of a species which is undergoing modification do, as a
matter of fact, exercise themselves in the same manner and to the same
extent. But the consideration of a definite example clearly shows that
this must be the case. When the wild duck became domesticated, and
lived in a farm-yard, all the individuals were compelled to walk and
stand more than they had done previously, and the muscles of the legs
were used to a correspondingly greater degree. The same thing happens
in the wild state, when any change in the conditions of life compels an
organ to be more largely used. No individual will be able to entirely
avoid this extra use, and each will endeavour to accommodate itself to
the new conditions according to its power. The amount of this power
depends upon the predisposition of the germ; and natural selection,
while it apparently decides between individuals of various degrees of
strength, is in truth operating upon the stronger and weaker germs.

But the very conclusions which have been drawn from the increase of
activity which has arisen from exercise, must also be drawn from the
instances of atrophy or degeneration following from the disuse of
organs.

Darwin long ago called attention to the fact that the degeneration of
an organ may, under certain circumstances, be beneficial to the
species. For example, he first proved in the instance of Madeira, that
the loss of wings may be of advantage to many beetles inhabiting
oceanic islands. The individuals with imperfectly developed or
atrophied wings have an advantage, because they are not carried out to
sea by the frequent winds. The small eyes, buried in fur, possessed by
moles and other subterranean mammals, can be similarly explained by
means of natural selection. So also, the complete disappearance of the
limbs of snakes is evidently a real advantage to animals which creep
through narrow holes and clefts; and the degeneration of the wings in
the ostrich and penguin is, in part, explicable as a favourable
modification of the organ of flight into an organ for striking air or
water respectively.

But when the degeneration of disused organs confers no benefits upon
the individual, the explanation becomes less simple. Thus we find that
the eyes of animals which inhabit dark caves (such as insects, crabs,
fish, Amphibia, etc.) have undergone degeneration; yet this can hardly
be of direct advantage to the animals, for they could live quite as
well in the dark with well-developed eyes. But we are here brought into
contact with a very important aspect of natural selection, viz. the
power of conservation exerted by it. Not only does the survival of the
fittest select the best, but it also maintains it[45]. The struggle for
existence does not cease with the foundation of a new specific type, or
with some perfect adaptation to the external or internal conditions of
life, but it becomes, on the contrary, even more severe, so that the
most minute differences of structure determine the issue between life
and death.

The sharpest sight possessed by birds is found in birds of prey, but if
one of them entered the world with eyes rather below the average in
this respect, it could not, in the long run, escape death from hunger,
because it would always be at a disadvantage as compared with others.

Hence the sharp sight of these birds is maintained by means of the
continued operation of natural selection, by which the individuals with
the weakest sight are being continually exterminated. But all this
would be changed at once, if a bird of prey of a certain species were
compelled to live in absolute darkness. The quality of the eyes would
then be immaterial, for it could make no difference to the existence of
the individual, or the maintenance of the species. The sharp sight
might, perhaps, be transmitted through numerous generations; but when
weaker eyes arose from time to time, these would also be transmitted,
for even very short-sighted or imperfect eyes would bring no
disadvantage to their owner. Hence, by continual crossing between
individuals with the most varied degrees of perfection in this respect,
the average of perfection would gradually decline from the point
attained before the species lived in the dark.

We do not at present know of any bird living in perfect darkness, and
it is improbable that such a bird will ever be found; but we are
acquainted with blind fish and Amphibia, and among these the eyes are
present it is true, but they are small and hidden under the skin. I
think it is difficult to reconcile the facts of the case with the
ordinary theory that the eyes of these animals have simply degenerated
through disuse. If disuse were able to bring about the complete atrophy
of an organ, it follows that every trace of it would be effaced. We
know that, as a matter of fact, the olfactory organ of the frog
completely degenerates when the olfactory nerve is divided; and that
great degeneration of the eye may be brought about by the artificial
destruction of the optic centre in the brain. Since, therefore, the
effects of disuse are so striking in a single life, we should certainly
expect, if such effects can be transmitted, that all traces of an eye
would soon disappear from a species which lives in the dark.

The caverns in Carniola and Carinthia, in which the blind _Proteus_ and
so many other blind animals live, belong geologically to the Jurassic
formation; and although we do not exactly know when for example the
_Proteus_ first entered them, the low organization of this amphibian
certainly indicates that it has been sheltered there for a very long
period of time, and that thousands of generations of this species have
succeeded one another in the caves.

Hence there is no reason to wonder at the extent to which the
degeneration of the eye has been already carried in the _Proteus_; even
if we assume that it is merely due to the cessation of the conserving
influence of natural selection.

But it is unnecessary to depend upon this assumption alone, for when a
useless organ degenerates, there are also other factors which demand
consideration, namely, the higher development of other organs which
compensate for the loss of the degenerating structure, or the increase
in size of adjacent parts. If these newer developments are of advantage
to the species, they finally come to take the place of the organ which
natural selection has failed to preserve at its point of highest
perfection.

In the first place, a certain form of correlation, which Roux[46] calls
‘the struggle of the parts in the organism,’ plays a most important
part. Cases of atrophy, following disuse, appear to be always attended
by a corresponding increase of other organs: blind animals always
possess very strongly developed organs of touch, hearing, and smell,
and the degeneration of the wing-muscles of the ostrich is accompanied
by a great increase in the strength of the muscles of the leg. If the
average amount of food which an animal can assimilate every day remains
constant for a considerable time, it follows that a strong influx
towards one organ must be accompanied by a drain upon others, and this
tendency will increase, from generation to generation, in proportion to
the development of the growing organ, which is favoured by natural
selection in its increased blood-supply, etc.; while the operation of
natural selection has also determined the organ which can bear a
corresponding loss without detriment to the organism as a whole.

Without the operation of natural selection upon different individuals,
the struggle between the organs of a single individual would be unable
to encourage a predisposition in the germ towards the degeneration or
non-development of a useless organ, and it could only limit and degrade
the development of an organ in the lifetime of the individual. If,
therefore, acquired characters are not transmitted, the disposition to
develope such an organ would be present in the same degree in each
successive generation, although the realization would be less perfect.
The complete disappearance of a rudimentary organ can only take place
by the operation of natural selection; this principle will lead to its
elimination, inasmuch as the disappearing structure takes the place and
the nutriment of other useful and important organs. Hence the process
of natural selection tends to entirely remove the former. The
predisposition towards a weaker development of the organ is thus
advantageous, and there is every reason for the belief that the
advantages would continue to be gained, and that therefore the
processes of natural selection would remain in operation, until the
germ had entirely lost all tendency towards the development of the
organ in question. The extreme slowness with which this process takes
place, and the extraordinary persistence of rudimentary organs, at any
rate in the embryo, together with their gradual but finally complete
disappearance, can be clearly seen in the limbs of certain vertebrates
and arthropods. The blind-worms have no limbs, but a rudimentary
shoulder-girdle is present close under the skin, and the interesting
fact has been quite recently established[47] that the fore-limbs are
present in the embryo in the form of short stumps, which entirely
disappear at a later stage. In most snakes all traces of limbs have
been lost in the adult, but we do not yet know for certain whether they
are also wanting in the embryo. I might further mention the very
different stages of degeneration witnessed in the limbs of various
salamanders; and the anterior limbs of _Hesperornis_—the remarkable
toothed bird from the cretaceous rocks—which, according to Marsh[48],
consists only of a very thin and relatively small humerus, which was
probably concealed beneath the skin. The water-fleas (_Daphnidae_)
possess in the embryonic state three complete and almost equal pairs of
jaws, but two of these entirely disappear, and do not develope into
jaws in any species. In the same way, the embryo of the maggot-like
legless larva of bees and wasps possesses three pairs of ancestral
limbs.

There are, however, cases in which, apparently, acquired variations of
characters are transmitted without natural selection playing any active
part in the change. Such a case is afforded by the short-sightedness so
common in civilized nations.

This affection is certainly hereditary in some cases, and it may well
have been explained as an example of the transmission of acquired
changes. It has been argued that acquired short-sightedness can be in a
slight degree transmitted, and that each successive generation has
developed a further degree of the disease by habitually holding books
etc. close to the eyes, so that the inborn predisposition to
short-sightedness is continually accumulating.

But we must remember that variations in the refraction of the human eye
have been for a long time independent of the preserving control of
natural selection. In the struggle for existence, a blind man would
certainly disappear before those endowed with sight, but myopia does
not prevent any one from gaining a living.

A short-sighted lynx, hawk, or gazelle, or even a short-sighted Indian,
would be eliminated by natural selection, but a short-sighted European
of the higher class finds no difficulty in earning his bread.

Those fluctuations on either side of the average which we call myopia
and hypermetropia, occur in the same manner, and are due to the same
causes, as those which operate in producing degeneration in the eyes of
cave-dwelling animals. If, therefore, we not infrequently meet with
families in which myopia is hereditary, such results may be attributed
to the transmission of an accidental disposition on the part of the
germ, instead of to the transmission of acquired short-sightedness. A
very large proportion of short-sighted people do not owe their
affliction to inheritance at all, but have acquired it for themselves;
for there is no doubt that a normal eye may be rendered myopic in the
course of a life-time by continually looking at objects from a very
short distance, even when no hereditary predisposition towards the
disease can be shown to exist. Such a change would of course appear
more readily if there was also a corresponding predisposition on the
part of the eye. But I should not explain this widely spread
predisposition towards myopia as due to the transmission of acquired
short-sightedness, but to the greater variability of the eye, which
necessarily results from the cessation of the controlling influence of
natural selection.

This suspension of the preserving influence of natural selection may be
termed _Panmixia_, for all individuals can reproduce themselves and
thus stamp their characters upon the species, and not only those which
are in all respects, or in respect to some single organ, the fittest.
In my opinion, the greater number of those variations which are usually
attributed to the direct influence of external conditions of life, are
to be ascribed to panmixia. For example, the great variability of most
domesticated animals essentially depends upon this principle.

A goose or a duck must possess strong powers of flight in the natural
state, but such powers are no longer necessary for obtaining food when
it is brought into the poultry-yard, so that a rigid selection of
individuals with well-developed wings, at once ceases among its
descendants. Hence in the course of generations, a deterioration of the
organs of flight must necessarily ensue, and the other members and
organs of the bird will be similarly affected.

This example very clearly indicates that the degeneration of an organ
does not depend upon its disuse; for although our domestic poultry very
rarely make use of their wings, the muscles of flight have not
disappeared, and, at any rate in the goose, do not seem to have
undergone any marked degeneration.

The numerous and exact observations conducted by Darwin upon the weight
and measurement of the bones in domestic fowls, seem to me to possess a
significance beyond that which he attributed to them.

If the weight of the wing-bones of the domestic duck bears a smaller
proportion to the weight of the leg-bones than in the wild duck, and
if, as Darwin rightly assumes, this depends not only upon the
diminution of the wings, but also upon the increase of the legs, it by
no means follows that this latter increase in organs which are now more
frequently used, is dependent upon hereditary influences alone.

It is quite possible that it depends, on the one hand, upon the
suspension of natural selection, or panmixia (and these effects would
be transmitted), and on the other hand upon the direct influence of
increased use during the course of a single life. We do not yet know
with any accuracy, the amount of change which may be produced by
increased use in the course of a single life. If it is desired to prove
that use and disuse produce hereditary effects without the assistance
of natural selection, it will be necessary to domesticate wild animals
(for example the wild duck) and preserve all their descendants, thus
excluding the operation of natural selection. If then all individuals
of the second, third, fourth and later generations of these tame ducks
possess identical variations, which increase from generation to
generation, and if the nature of these changes proves that they must
have been due to the effect of use or disuse, then perhaps the
transmission of such effects may be admitted; but it must always be
remembered that domestication itself influences the organism,—not only
directly, but also indirectly, by the increase of variability as a
result of the suspension of natural selection. Such experiments have
not yet been carried out in sufficient detail[49].

It is usually considered that the origin and variation of instincts are
also dependent upon the exercise of certain groups of muscles and
nerves during a single life-time; and that the gradual improvement
which is thus caused by practice, is accumulated by hereditary
transmission. I believe that this is an entirely erroneous view, and I
hold that all instinct is entirely due to the operation of natural
selection, and has its foundation, not upon inherited experiences, but
upon the variations of the germ.

Why, for instance, should not the instinct to fly from enemies have
arisen by the survival of those individuals which are naturally timid
and easily startled, together with the extermination of those which are
unwary? It may be urged in opposition to this explanation that the
birds of uninhabited islands which are not at first shy of man, acquire
in a few generations an instinctive dread of him, an instinct which
cannot have arisen in so short a time by means of natural selection.
But, in this case are we really dealing with the origin of a new
instinct, or only with the addition of one new perception
(‘Wahrnehmung,’ Schneider)[50], of the same kind as those which incite
to the instinct of flight—an instinct which had been previously
developed in past ages but had never been called forth by man? Again,
has any one ascertained whether the young birds of the second or third
generation are frightened by man? May it not be that the experience of
a single life-time plays a great part in the origin of the habit? For
my part, I am inclined to believe that the habit of flying from man is
developed in the first generation which encounters him as a foe[51]. We
see how wary and cautious a flock of birds become as soon as a few
shots have been fired at them, and yet shortly before this occurrence
they were perhaps playing carelessly close to the sportsmen.
Intelligence plays a considerable part in the life of birds, and it by
no means follows that the transmission of individual habits explains
the above-mentioned phenomena. The long-continued operation of natural
selection may very well have been necessary before the perception of
man could awake the instinct to flee in young, inexperienced birds.
Unfortunately the observations upon these points are far too indefinite
to enable us to draw conclusions.

There is again the frequently-quoted instance of the young pointer,
‘which, untrained, and without any example which might have been
imitated, pointed at a lizard in a subtropical jungle, just as many of
its forefathers had pointed at partridges on the plain of St. Denis,’
and which, without knowing the effect of a shot, sprang forward
barking, at the first discharge, to bring in the game. This conduct
must not be attributed to the inheritance of any mental picture, such
as the effect of a shot, but to the inheritance of a certain reflex
mechanism. The young pointer does not spring forward at the shot
because he has inherited from his forefathers a certain association of
ideas,—shot and game,—but because he has inherited a reflex mechanism,
which impels him to start forward on hearing a report. We cannot yet
determine without more experiments how such an impulse due to
perception (‘Wahrnehmungstrieb,’ Schneider) has arisen; but, in my
opinion, it is almost inconceivable that artificial breeding has had
nothing to do with it; and that we are here concerned—not with the
inheritance of the effects of training—but with some predisposition on
the part of the germ, which has been increased by artificial selection.

The necessity for extreme caution in appealing to the supposed
hereditary effects of use, is well shown in the case of those numerous
instincts, which only come into play once in a lifetime, and which do
not therefore admit of improvement by practice. The queen-bee takes her
nuptial flight only once, and yet how many and complex are the
instincts and the reflex mechanisms which come into play on that
occasion. Again, in many insects the deposition of eggs occurs but once
in a life-time, and yet such insects always fulfil the necessary
conditions with unfailing accuracy, either simply dropping the eggs
into water, or carefully fixing them on the surface of the earth
beneath some stone, or laying them on a particular part of a certain
species of plant; and in all these cases the most complicated actions
are performed. It is indeed astonishing to watch one of the _Cynipidae_
(_Rhodites rosae_) depositing her eggs in the tissue of a young bud.
She first carefully examines the bud on all sides, and feels it with
her legs and antennae. Then she slowly inserts her long ovipositor
between the closely-rolled leaves of the bud, but if it does not reach
exactly the right spot, she will withdraw and re-insert it many times,
until at length, when the proper place has been found, she will slowly
bore deep into the very centre of the bud, so that the egg will reach
the exact spot, where the necessary conditions for its development
alone exist.

But each _Cynips_ lays eggs many times, and it may be argued that
practice may have led to improvement in this case; we cannot however,
as a matter of fact, expect much improvement in a process which is
repeated, perhaps a dozen times, at short intervals of time, and which
is of such an excessively complex nature.

It is the same with the deposition of eggs in most insects. How can
practice have had any influence upon the origin of the instinct which
leads one of our butterflies—(_Vanessa levana_)—to lay its green eggs
in single file, as columns, which project freely from the stem or leaf,
so that protection is gained by their close resemblance to the
flower-buds of the stinging-nettle, which forms the food-plant of their
caterpillars?

Of course the butterfly is not aware of the advantage which follows
from such a proceeding; intelligence has no part in the process. The
entire operation depends upon certain inherent anatomical and
physiological arrangements:—on the structure of the ovary and oviducts,
on the simultaneous ripening of a certain number of eggs, and on
certain very complex reflex mechanisms which compel the butterfly to
lay its eggs on certain parts of certain plants. Schneider is certainly
right when he maintains that this mechanism is released by a sensation,
arising from the perception (whether by sight or smell, or both
together) of the particular plant or part of the plant upon which the
eggs are to be laid[52]. At any rate, we cannot, in such cases, appeal
to the effects of constant use and the transmission of acquired
characters, as an explanation; and the origin of the impulse can only
be understood as a result of the process of natural selection.

The protective cocoons by which the pupae of many insects are
surrounded also belong to the same category, and improvement by
practice is entirely out of the question, for they are only constructed
once in the course of a life-time. And yet these cocoons are often
remarkably complex: think, for instance, of the cocoon spun by the
caterpillar of the emperor moth (_Saturnia carpini_), which is so tough
that it can hardly be torn, and which the moth would be unable to
leave, if an opening were not provided for the purpose; while, on the
other hand, the pupa would not be defended against enemies if the
opening were not furnished with a circle of pointed bristles,
converging outwards, on the principle of the lobster pot, so that the
moth can easily emerge, although no enemy can enter. The impulse which
leads to the production of such a structure can only have arisen by the
operation of natural selection—not, of course, during the history of a
single species, but during the development of numerous, consecutive
species—by gradual and unceasing improvements in the initial stages of
cocoon-building. A number of species exists at the present day, of
which the cocoons can be arranged in a complete series, becoming
gradually less and less complex, from that described above, down to a
loosely-constructed, spherical case in which the pupa is contained.

The cocoon spun by the larva of _Saturnia carpini_ differs but little
in complexity from the web of the spider, and if the former is
constructed without assistance from the experience of the single
individual—and this must certainly be admitted—it follows that the
latter may be also built without the aid of experience, while there is
neither reason nor necessity for appealing to the entirely unproved
transmission of acquired skill in order to explain this and a thousand
other operations.

It may be objected that, in man, in addition to the instincts inherent
in every individual, special individual predispositions are also found,
of such a nature that it is impossible that they can have arisen by
individual variations of the germ. On the other hand, these
predispositions—which we call talents—cannot have arisen through
natural selection, because life is in no way dependent upon their
presence, and there seems to be no way of explaining their origin
except by an assumption of the summation of the skill attained by
exercise in the course of each single life. In this case, therefore, we
seem at first sight to be compelled to accept the transmission of
acquired characters.

Now it cannot be denied that all predispositions may be improved by
practice during the course of a life-time,—and, in truth, very
remarkably improved. If we could explain the existence of great talent,
such as, for example, a gift for music, painting, sculpture, or
mathematics, as due to the presence or absence of a special organ in
the brain, it follows that we could only understand its origin and
increase (natural selection being excluded) by accumulation, due to the
transmission of the results of practice through a series of
generations. But talents are not dependent upon the possession of
special organs in the brain. They are not simple mental dispositions,
but combinations of many dispositions, and often of a most complex
nature: they depend upon a certain degree of irritability, and a power
of readily transmitting impulses along the nerve-tracts of the brain,
as well as upon the especial development of single parts of the brain.
In my opinion, there is absolutely no trustworthy proof that talents
have been improved by their exercise through the course of a long
series of generations. The Bach family shows that musical talent, and
the Bernoulli family that mathematical power, can be transmitted from
generation to generation, but this teaches us nothing as to the origin
of such talents. In both families the high-water mark of talent lies,
not at the end of the series of generations, as it should do if the
results of practice are transmitted, but in the middle. Again, talents
frequently appear in some single member of a family which has not been
previously distinguished.

Gauss was not the son of a mathematician; Handel’s father was a
surgeon, of whose musical powers nothing is known; Titian was the son
and also the nephew of a lawyer, while he and his brother, Francesco
Vecellio, were the first painters in a family which produced a
succession of seven other artists with diminishing talents. These facts
do not, however, prove that the condition of the nerve-tracts and
centres of the brain, which determine the specific talent, appeared for
the first time in these men: the appropriate condition surely existed
previously in their parents, although it did not achieve expression.
They prove, as it seems to me, that a high degree of endowment in a
special direction, which we call talent, cannot have arisen from the
experience of previous generations, that is, by the exercise of the
brain in the same specific direction.

It appears to me that talent consists in a happy combination of
exceptionally high gifts, developed in one special direction. At
present, it is of course impossible to understand the physiological
conditions which render the origin of such combinations possible, but
it is very probable that the crossing of the mental dispositions of the
parents plays a great part in it. This has been admirably and concisely
expressed by Goethe in describing his own characteristics—

                     Vom Vater hab’ ich die Statur
                     Des Lebens ernstes Führen,
                     Vom Mütterchen die Frohnatur
                     Die Lust zum Fabuliren, etc.

The combination of talents frequently found in one individual, and the
appearance of different remarkable talents in the various branches of
one and the same family, indicate that talents are only special
combinations of certain highly-developed mental dispositions which are
found in every brain. Many painters have been admirable musicians, and
we very frequently find both these talents developed to a slighter
extent in a single individual. In the Feuerbach family we find a
distinguished jurist, a remarkable philosopher, and a highly-talented
artist; and among the Mendelssohns a philosopher as well as a musician.

The sudden and yet widespread appearance of a particular talent in
correspondence with the general intellectual excitement of a certain
epoch points in the same direction. How many poets arose in Germany
during the period of sentiment which marked the close of the last
century, and how completely all poetic gifts seem to have disappeared
during the Thirty Years’ War. How numerous were the philosophers that
appeared in the epoch which succeeded Kant; while all philosophic
talent seemed to have deserted the German nation during the sway of the
antagonistic ‘exact science,’ with its contempt for speculation.

Wherever academies are founded, there the Schwanthalers, Defreggers,
and Lenbachs emerge from the masses which had shown no sign of artistic
endowment through long periods of time[53]. At the present day there
are many men of science who, had they lived at the time of Bürger,
Uhland, or Schelling, would probably have been poets or philosophers.
And the man of science also cannot dispense with that mental
disposition directed in a certain course, which we call talent,
although the specific part of it may not be so obvious: we may, indeed,
go further, and maintain that the Physicist and the Chemist are
characterized by a combination of mental dispositions which differ from
those of the Botanist and the Zoologist. Nevertheless, a man is not
born a physicist or a botanist, and in most cases chance alone
determines whether his endowments are developed in either direction.

Lessing has asked whether Raphael would have been a less distinguished
artist had he been born without hands: we might also enquire whether he
might not have been as great a musician as he was painter if, instead
of living during the historical high-water mark of painting, he had
lived, under favourable personal influences, at the time of
highly-developed and widespread musical genius. A great artist is
always a great man, and if he finds the outlet for his talent closed on
one side, he forces his way through on the other.

From all these examples I wish to show that, in my opinion, talents do
not appear to depend upon the improvement of any special mental quality
by continued practice, but they are the expression, and to a certain
extent the bye-product, of the human mind, which is so highly developed
in all directions.

But if any one asks whether this high mental development, acquired in
the course of innumerable generations of men, is not dependent upon the
hereditary effects of use, I would remind him that human intelligence
in general is the chief means and the chief weapon which has served and
still serves the human species in the struggle for existence[54]. Even
in the present state of civilization—distorted as it is by numerous
artificial encroachments and unnatural conditions—the degree of
intelligence possessed by the individual chiefly decides between
destruction and life; and in a natural state, or still better in a
state of low civilization, this result is even more striking.

Here again, therefore, we encounter the effects of natural selection,
and to this power we must attribute, at any rate, a great part of the
phenomena we have been discussing, and it cannot be shown that—in
addition to its operation—the transmission of characters acquired by
practice plays any part in nature.

I only know of one class of changes in the organism which is with
difficulty explained by the supposition of changes in the germ; these
are the modifications which appear as the direct consequence of some
alteration in the surroundings. But our knowledge on this subject is
still very defective, and we do not know the facts with sufficient
precision to enable us to pronounce a final verdict as to the cause of
such changes: and for this reason, I do not propose to consider the
subject in detail.

These changes—such, for example, as are produced by a strange
climate—have been always looked at under the supposition that they are
transmitted and intensified from generation to generation, and for this
reason the observations are not always sufficiently precise. It is
difficult to say whether the changed climate may not have first changed
the germ, and if this were the case the accumulation of effects through
the action of heredity would present no difficulty. For instance, it is
well known that increased nourishment not only causes a plant to grow
more luxuriantly, but it alters the plant in some distinct way, and it
would be wonderful indeed if the seeds were not also larger and better
furnished with nutritive material. If the increased nourishment be
repeated in the next generation, a still further increase in the size
of the seed, in the luxuriance of the plant, and in all other changes
which ensue, is at any rate conceivable if it is not a necessity. But
this would not be an instance of the transmission of acquired
characters, but only the consequence of a direct influence upon the
germ-cells, and of better nourishment during growth.

A similar interpretation explains the converse change. When horses of
normal size are introduced into the Falkland Islands, the next
generation is smaller in consequence of poor nourishment and the damp
climate, and after a few generations they have deteriorated to a marked
extent. In such a case we have only to assume that the climate which is
unfavourable and the nutriment which is insufficient for horses, affect
not only the animal as a whole, but also its germ-cells. This would
result in the diminution in size of the germ-cells, the effects upon
the offspring being still further intensified by the insufficient
nourishment supplied during growth. But such results would not depend
upon the transmission by the germ-cells of certain peculiarities due to
the unfavourable climate, which only appear in the full-grown horse.

It must be admitted that there are cases, such as the climatic
varieties of certain butterflies, which raise some difficulties against
this explanation. I myself, some years ago, experimentally investigated
one such case[55], and even now I cannot explain the facts otherwise
than by supposing the passive acquisition of characters produced by the
direct influence of climate.

It must be remembered, however, that my experiments, which have been
repeated upon several American species by H. W. Edwards, with results
confirmatory of my own in all essential respects, were not undertaken
with the object of investigating the question from this point of view
alone. New experiments, under varying conditions, will be necessary to
afford the true explanation of this aspect of the question; and I have
already begun to undertake them.

Leaving on one side, for the moment, these doubtful, and insufficiently
investigated cases, we may still maintain that the assumption that
changes induced by external conditions in the organism as a whole, are
communicated to the germ-cells after the manner indicated in Darwin’s
hypothesis of pangenesis,—is wholly unnecessary for the explanation of
these phenomena. Still we cannot exclude the possibility of such a
transmission occasionally occurring, for, even if the greater part of
the effects must be attributed to natural selection, there might be a
smaller part in certain cases which depends on this exceptional factor.

A complete and satisfactory refutation of such an opinion cannot be
brought forward at present: we can only point out that such an
assumption introduces new and entirely obscure forces, and that
innumerable cases exist in which we can certainly exclude all
assistance from the transmission of acquired characters. In most cases
of variation in colour we have no explanation but the survival of the
fittest[56], and the same holds good for all changes of form which
cannot be influenced by the will of the animal. Very numerous
adaptations, such, for instance, as occur in the eggs of animals,—the
markings, and appendages which conceal them from enemies, the complex
coverings which prevent them from drying up or protect them from the
injurious influence of cold,—must have all arisen entirely
independently of any expression of will, or of any conscious or
unconscious action on the part of the animals. I will not mention here
the case of plants, which as every one knows are unconscious, for they
are beyond my province. In this matter, there can be no suggestion of
adaptation depending upon a struggle between the various parts of the
organism (Roux)[57]. Natural selection cannot operate upon the
different epithelial cells which secrete the egg-shell of _Apus_, since
it is of no consequence to the animal which secretes the egg-shell
whether a good or a bad shell is produced. Natural selection first
operates among the offspring, and the egg with a shell incapable of
resisting cold or drought is destroyed. The different cells of the same
individual are not selected, but the different individuals themselves.

In all such cases we have no explanation except the operation of
natural selection, and if we cannot accept this, we may as well abandon
any attempt at a natural explanation. But, in my opinion, there is no
reason why natural selection should be considered inadequate to the
task. It is true that the objection has been lately urged, that it is
inconceivable that all the wonderful adaptations of the organism to its
surroundings can have arisen through the selection of individuals; and
that for this purpose an infinite number of individuals and infinite
time would be required; and stress is laid upon the fact that the
wished-for useful changes can only arise singly and very rarely among a
great number of individuals.

This last objection to the modern conception of natural selection has
apparently some weight, for, as a matter of fact, useful variations of
a conspicuous kind seldom appear, and are often entirely absent for
many generations. If we expect to find that qualitative changes take
place by sudden leaps, we can never escape this difficulty. But, I
think, we must not look for conspicuous variations—such as occur among
domesticated animals and plants—in the process of the evolution of
species as it goes on in nature. Natural selection does not deal with
qualitative but quantitative changes in the individual, and the latter
are always present.

A simple example will make this clearer. Let us suppose that it was
advantageous to some species—for instance the ancestors of the
giraffe—to lengthen some part of the body, such as the neck: this
result could be obtained in a relatively short time, for the members of
the species already possessed necks of varying length, and the
variations which form the material for natural selection were already
in existence. Now all the organs of every species vary in size, and any
one of them will undergo constant and progressive increase, as soon as
it acquires exceptional usefulness. But not only will the organ
fluctuate as a whole, but also the parts composing it will become
larger or smaller under given conditions, will increase or diminish by
the operation of natural selection. I believe that qualitative
variations always depend upon differences in the size and number of the
component parts of the whole. A skin appears to be naked, when it is
really covered with a number of small fine hairs: if these grow larger
and increase in number, a thick covering is formed, and we say that the
skin is woolly or furry. In the same way the skin of many worms and
Crustacea is apparently colourless, but the microscope reveals the
presence of a number of beautiful pigment spots; and not until these
have increased enormously does the skin appear coloured to the naked
eye. The presence or absence of colour and its quality when present are
here dependent upon the quantity of the most minute particles, and on
the distance at which the object in question is observed. Again, the
first appearance of colour, or the change from a green to a yellow or
red colour depends upon slight variations in the position or in the
number of the oxygen atoms which enter into the chemical combination in
question. Fluctuations in the chemical composition of the molecules of
a unicellular organism (for example) must continually arise, just as
fluctuations are always occurring in the number of pigment granules in
a certain cell, or in the number of pigment cells in a certain region
of the body, or even in the size of the various parts of the body.

All these quantitative relations are exposed to individual fluctuations
in every species; and natural selection can strengthen the fluctuations
of any part, and thus cause it to develope further in any given
direction.

From this point of view, it becomes less astonishing and less
inconceivable that organisms adapt themselves—as we see that they
obviously do—in all their parts to any condition of existence, and that
they behave like a plastic mass which can be moulded into almost any
imaginable form in the course of time.

If we ask in what lies the cause of this variability, the answer must
undoubtedly be that it lies in the germ-cells. From the moment when the
phenomena which precede segmentation commence in the egg, the exact
kind of organism which will be developed is already determined—whether
it will be larger or smaller, more like its father or its mother, which
of its parts will resemble the one and which the other, even to the
minutest detail. In spite of this, there still remains a certain scope
for the influence of external conditions upon the organism. But this
scope is limited, and forms but a small area round the fixed central
point which is determined by heredity. Abundant nourishment can make
the body large and strong, but can never make a giant out of the
germ-cell destined to become a dwarf. Unhealthy sedentary habits or
insufficient nourishment makes the factory-hand pale and stunted; life
on board ship, with plenty of exercise and sea air, gives the sailor
bodily strength and a tanned skin; but when once the resemblance to
father or mother, or to both, is established in the germ-cell it can
never be effaced, let the habit of life be what it will.

But if the essential nature of the germ-cell dominates over the
organism which will grow from it, so also the quantitative individual
differences, to which I referred just now, are, by the same principle,
established in the germ, and—whatever be the cause which determines
their presence—they must be looked upon as inherent in it. It therefore
follows that, although natural selection appears to operate upon the
qualities of the developed organism alone, it in truth works upon
peculiarities which lie hidden in the germ-cells. Just as the final
development of any predisposition in the germ, and just as any
character in the mature organism vibrates with a certain amplitude
around a fixed central point, so the predisposition of the germ itself
fluctuates, and it is on this that the possibility of an increase of
the predisposition in question, and its average result, depends.

If we trace all the permanent hereditary variations from generation to
generation back to the quantitative variations of the germ, as I have
sought to do, the question naturally occurs as to the source from which
these variations arose in the germ itself. I will not enter into this
subject at any length on the present occasion, for I have already
expressed my opinion upon it[58].

I believe however that they can be referred to the various external
influences to which the germ is exposed before the commencement of
embryonic development. Hence we may fairly attribute to the adult
organism influences which determine the phyletic development of its
descendants. For the germ-cells are contained in the organism, and the
external influences which affect them are intimately connected with the
state of the organism in which they lie hid. If it be well nourished,
the germ-cells will have abundant nutriment; and, conversely, if it be
weak and sickly, the germ-cells will be arrested in their growth. It is
even possible that the effects of these influences may be more
specialized; that is to say, they may act only upon certain parts of
the germ-cells. But this is indeed very different from believing that
the changes of the organism which result from external stimuli can be
transmitted to the germ-cells and will re-develope in the next
generation at the same time as that at which they arose in the parent,
and in the same part of the organism.

We have an obvious means by which the inheritance of all transmitted
peculiarities takes place, in _the continuity of the substance of the
germ-cells, or germ-plasm_. If, as I believe, the substance of the
germ-cells, the germ-plasm, has remained in perpetual continuity from
the first origin of life, and if the germ-plasm and the substance of
the body, the somatoplasm, have always occupied different spheres, and
if changes in the latter only arise when they have been preceded by
corresponding changes in the former, then we can, up to a certain
point, understand the principle of heredity; or, at any rate, we can
conceive that the human mind may at some time be capable of
understanding it. We may at least maintain that it has been rendered
intelligible, for we can thus trace heredity back to growth; we can
thus look upon reproduction as an overgrowth of the individual, and can
thus distinguish between a succession of species and a succession of
individuals, because in the latter succession the germ-plasm remains
similar, while in the succession of the former it becomes different.
Thus individuals, as they arise, are always assuming new and more
complex forms, until the interval between the simple unicellular
protozoon and the most complex of all organisms—man himself—is bridged
over.

I have not been able to throw light upon all sides of the question
which we are here discussing. There are still some essential points
which I must leave for the present; and, furthermore, I am not yet in a
position to explain satisfactorily all the details which arise at every
step of the argument. But it appeared to me to be necessary to state
this weighty and fundamental question, and to formulate it concisely
and definitely; for only in this way will it be possible to arrive at a
true and lasting solution of the problem. We must however be clear on
this point—that the understanding of the phenomena of heredity is only
possible on the fundamental supposition of the continuity of the
germ-plasm. The value of experiment in relation to this question is
somewhat doubtful. A careful collection and arrangement of facts is far
more likely to decide whether, and to what extent, the continuity of
germ-plasm is reconcilable with the assumption of the transmission of
acquired characters from the parent body to the germ, and from the germ
to the body of the offspring. At present such transmission is neither
proved as a fact, nor has its assumption been shown to be
unquestionably necessary.

------------------------------------------------------------------------

                        Footnotes for Essay II.

Footnote 33:

  Pflüger, ‘Ueber den Einfluss der Schwerkraft auf die Theilung der
  Zellen und auf die Entwicklung des Embryo,’ Arch. f. Physiol. Bd.
  XXXII. p. 68, 1883.

Footnote 34:

  Victor Hensen in his ‘Physiologie der Zeugung,’ Leipzig, 1881, p. 216.

Footnote 35:

  That is for the preservation of its life.

Footnote 36:

  Compare Weismann, ‘Die Entstehung der Sexualzellen bei den
  Hydromedusen,’ Jena, 1883.

Footnote 37:

  It is doubtful whether _Magosphaera_ should be looked upon as a
  mature form; but nothing hinders us from believing that species have
  lived, and are still living, in which the ciliated sphere has held
  together until the encystment, that is the reproduction, of the
  constituent single cells.

Footnote 38:

  Or is an exception perhaps afforded by the nutritive cells of the
  egg, which occur in many animals?

Footnote 39:

  Or more precisely, they must give up as many molecules as would
  correspond to the number of the kind of cell in question found in the
  mature organism.

Footnote 40:

  See Darwin, ‘The Variation of Animals and Plants under
  Domestication,’ 1875, vol. ii. chapter xxvii. pp. 349-399.

Footnote 41:

  To this class of phenomena of course belong those acts of will which
  call forth the functional activity of certain groups of cells. It is
  quite clear that such impulses do not originate in the constitution
  of the tissue in question, but are due to the operation of external
  causes. The activity does not arise directly from any natural
  disposition of the germ, but is the result of accidental external
  impressions. A domesticated duck uses its legs in a different manner
  from, and more frequently than a wild duck, but such functional
  changes are the consequence of changed external conditions, and are
  not due to the constitution of the germ.

Footnote 42:

  Upon this subject Pflüger states—‘I have made myself accurately
  acquainted with all facts which are supposed to prove the inheritance
  of acquired characters,—that is of characters which are not due to
  the peculiar organization of the ovum and spermatozoon from which the
  individual is formed, but which follow from the incidence of
  accidental external influences upon the organism at any time in its
  life. Not one of these facts can be accepted as a proof of the
  transmission of acquired characters.’ _l. c._ p. 68.

Footnote 43:

  ‘Physiologie der Zeugung.’

Footnote 44:

  See ‘Ueber die Uebung,’ Berlin, 1881.

Footnote 45:

  This principle was, I believe, first pointed out by Seidlitz. Compare
  Seidlitz, ‘Die Darwin’sche Theorie,’ Leipzig, 1875, p. 198.

Footnote 46:

  W. Roux, ‘Der Kampf der Theile im Organismus,’ Leipzig, 1881.

Footnote 47:

  Compare Born in ‘Zoolog. Anzeiger,’ 1883, No. 150, p. 537.

Footnote 48:

  O. C. Marsh, ‘Odontornithes, a Monograph on the extinct toothed Birds
  of North America,’ Washington, 1880.

Footnote 49:

  C. Darwin, ‘Variation of Animals and Plants under Domestication.’
  Vol. I.

Footnote 50:

  Compare ‘Der thierische Wille,’ Leipzig, 1880.

Footnote 51:

  Steller’s interesting account of the Sea-cow (_Rhytina Stelleri_)
  proves that this suggestion is valid. This large mammal was living in
  great numbers in Behring Strait at the end of the last century, but
  has since been entirely exterminated by man. Steller, who was
  compelled by shipwreck to remain in the locality for a whole year,
  tells us that the animals were at first without any fear of man, so
  that they could be approached in boats and could thus be killed.
  After a few months however the survivors became wary, and did not
  allow Steller’s men to approach them, so that they were difficult to
  catch.—A. W., 1888.

Footnote 52:

  Compare Schneider, ‘Der thierische Wille.’

Footnote 53:

  [The author refers to the Academy of Arts at Munich. S. S.]

Footnote 54:

  Compare Darwin’s ‘Descent of Man.’

Footnote 55:

  ‘Studien zur Descendenztheorie, I. Ueber den Saison-Dimorphismus der
  Schmetterlinge.’ Leipzig, 1875. English edition translated and edited
  by Professor Meldola, ‘Studies in the Theory of Descent,’ Part I.

Footnote 56:

  The colours which have been called forth by sexual selection must
  also be included here.

Footnote 57:

  Wilhelm Roux, ‘Der Kampf der Theile im Organismus.’ Leipzig, 1881.

Footnote 58:

  Consult ‘Studien zur Descendenztheorie, IV. Über die mechanische
  Auffassung der Natur,’ p. 303, etc. Translated and edited by
  Professor Meldola; see ‘Studies in the Theory of Descent,’ p. 677, &c.

------------------------------------------------------------------------






                                  III.

                            LIFE AND DEATH.

                                 1883.




------------------------------------------------------------------------




                            LIFE AND DEATH.


                                PREFACE.

The following paper was first printed as an academic lecture in the
summer of the present year (1883), with the title ‘Upon the Eternal
Duration of Life’ (‘Über die Ewigkeit des Lebens’). In now bringing it
before a larger public in an expanded and improved form, I have chosen
a title which seemed to me to correspond better with the present
contents of the paper.

The stimulus which led to this biological investigation was given in a
memoir by Götte, in which this author opposes views which I had
previously expressed. Although such an origin has naturally caused my
paper to take the form of a reply, my intention was not merely to
controvert the views of my opponent, but rather—using those opposed
views as a starting-point—to throw new light upon certain questions
which demand consideration; to give additional support to thoughts
which I have previously expressed, and to penetrate, if possible, more
deeply into the problem of life and death.

If, in making this attempt, the views of my opponent have been severely
criticized, it will be acknowledged that the criticisms do not form the
purpose of my paper, but only the means by which the way to a more
correct understanding of the problems before us may be indicated.

                                                               A. W.

      Freiburg i. Breisgau,
          _Oct. 18, 1883_.

------------------------------------------------------------------------




                                  III.

                            LIFE AND DEATH.


In the previous essay, entitled ‘The Duration of Life,’ I have
endeavoured to show that the limitation of life in single individuals
by death is not, as has been hitherto assumed, an inevitable
phenomenon, essential to the very nature of life itself; but that it is
an adaptation which first appeared when, in consequence of a certain
complexity of structure, an unending life became disadvantageous to the
species. I pointed out that we could not speak of natural death among
unicellular animals, for their growth has no termination which is
comparable with death. The origin of new individuals is not connected
with the death of the old; but increase by division takes place in such
a way that the two parts into which an organism separates are exactly
equivalent one to another, and neither of them is older or younger than
the other. In this way countless numbers of individuals arise, each of
which is as old as the species itself, while each possesses the
capability of living on indefinitely, by means of division.

I suggested that the Metazoa have lost this power of unending life by
being constructed of numerous cells, and by the consequent division of
labour which became established between the various cells of the body.
Here also reproduction takes place by means of cell-division, but every
cell does not possess the power of reproducing the whole organism. The
cells of the organism are differentiated into two essentially different
groups, the reproductive cells—ova or spermatozoa, and the somatic
cells, or cells of the body, in the narrower sense. The immortality of
the unicellular organism has only passed over to the former; the others
must die, and since the body of the individual is chiefly composed of
them, it must die also.

I have endeavoured to explain this fact as an adaptation to the general
conditions of life. In my opinion life became limited in its duration,
not because it was contrary to its very nature to be unlimited, but
because an unlimited persistence of the individual would be a luxury
without a purpose. Among unicellular organisms natural death was
impossible, because the reproductive cell and the individual were one
and the same: among multicellular animals it was possible, and we see
that it has arisen.

Natural death appeared to me to be explicable on the principle of
utility, as an adaptation.

These opinions, to which I shall return in greater detail in a later
part of this paper, have been opposed by Götte[59], who does not
attribute death to utility, but considers it to be a necessity inherent
in life itself. He considers that it occurs not only in the Metazoa or
multicellular animals, but also in unicellular forms of life, where it
is represented by the process of encystment, which is to be regarded as
the death of the individual. This encystment is a process of
rejuvenescence, which, after a longer or shorter interval, interrupts
multiplication by means of fission. According to Götte, this process of
rejuvenescence consists in the dissolution of the specific structure of
the individual, or in the retrogression of the individual to a form of
organic matter which is no longer living but which is comparable to the
yolk of an egg. This matter is, by means of its internal energy, and in
consequence of the law of growth which is inherent in its constitution,
enabled to give rise to a new individual of the same species.
Furthermore, the process of rejuvenescence among unicellular beings
corresponds to the formation of germs in the higher organisms. The
phenomena of death were transmitted by heredity from the unicellular
forms to the Metazoa when they arose. Death does not therefore appear
for the first time in the Metazoa, but it is an extremely ancient
process which ‘goes back to the first origin of organic beings’ (l. c.,
p. 81).

It is obvious, from this short _résumé_, that Götte’s view is totally
opposed to mine. Inasmuch as only one of these views can be
fundamentally right, it is worth while to compare the two; and although
we cannot at present hope to explain the ultimate physiological
processes which involve life and death, I think nevertheless that it is
quite possible to arrive at definite conclusions as to the general
causes of these phenomena. At any rate, existing facts have not been so
completely thought out that it is useless to consider them once more.

The question—what do we understand by death? must be decided before we
can speak of the origin of death. Götte says, ‘we are not able to
explain this general expression quite definitely and in all its
details, because the moment of death, or perhaps more exactly the
moment when death is complete, can in no case be precisely indicated.
We can only say that in the death of the higher animals, all those
phenomena which make up the life of the individual cease, and further
that all the cells and elements of tissue which form the dead organism,
die, and are resolved into their elements.’

This definition would suffice if it did not include that which is to be
defined. For it assumes that under the expression ‘dead organism’ we
must include those organisms which have brought to an end the whole of
their vital functions, but of which the component cells and elements
may still be living. This view is afterwards more accurately explained,
and in fact there is no doubt that the cessation of the activity of
life in the multicellular organism rarely implies any direct connection
with the cessation of vital functions in all its constituents. The
question however arises, whether it is right or useful to limit the
conception of death to the cessation of the functions of the organism.
Our conceptions of death have been derived from the higher organisms
alone, and hence it is quite possible that the conception may be too
limited. The limitation might perhaps be removed by accurate and
scientific comparison with the somewhat corresponding phenomena among
unicellular organisms, and we might then arrive at a more comprehensive
definition. Science has without doubt the right to make use of popular
terms and conceptions, and by a more profound insight to widen or
restrict them. But the main idea must always be retained, so that
nothing quite new or strange may appear in the widened conception. The
conception of death, as it has been expressed with perfect uniformity
in all languages, has arisen from observations on the higher animals
alone; and it signifies not only the cessation of the vital functions
of the whole organism, but at the same time the cessation of life in
its single parts, as is shown by the impossibility of revival. The
_post-mortem_ death of the cells is also part of death, and was so,
long before science established the fact that an organism is built up
of numerous very minute living elements, of which the vital processes
partially continue for some time after the cessation of those of the
whole organism. It is precisely this incapacity on the part of the
organism to reproduce the phenomena of life anew, which distinguishes
genuine death from the arrest of life or trance; and the incapacity
depends upon the fact that the death of the cells and tissues follows
upon the cessation of the vital functions as a whole. I would, for this
reason, define death as an arrest of life, from which no lengthened
revival, either of the whole or any of its parts, can take place; or,
to put it concisely, as a definite arrest of life. I believe that in
this definition I have expressed the exact meaning of the conception
which language has sought to convey in the word death. For our present
purpose, the cause which gives rise to this phenomenon is of no
importance,—whether it is simultaneous or successive in the various
parts of the organism, whether it makes its appearance slowly or
rapidly. For the conception itself it is also quite immaterial whether
we are able to decide if death has really taken place in any particular
case; however uncertain we might be, the state which we call death
would be not less sharply and definitely limited. We might consider the
caterpillar of _Euprepia flavia_ to be dead when frozen in ice, but if
it recovered after thawing and became an imago, we should say that it
had only been apparently dead, that life stood still for a time, but
had not ceased for ever. It is only the irretrievable loss of life in
an organism which we call death, and we ought to hold fast to this
conception, so that it will not slip from us, and become worthless,
because we no longer know what we mean by it.

We cannot escape this danger if we look upon the _post-mortem_ death of
the cells of the body as a phenomenon which may accompany death, but
which may sometimes be wanting. An experiment might be made in which
some part of a dead animal, such as the comb of a cock, might be
transplanted, before the death of the cells, to some other living
animal: such a part might live in its new position, thus showing that
single members may survive after the appearance of death, as I
understand it. But the objection might be raised that in such a case
the cock’s comb has become a member of another organism, so that it
would be lost labour to insert a clause in our definition of death
which would include this phenomenon. The same objection might be raised
if the transplantation took place a day or even a year before the death
of the cock.

Götte is decidedly in error when he considers that the idea of death
merely expresses an ‘arrest of the sum of vital actions in the
individual,’ without at the same time including that definite arrest
which involves the impossibility of any revival. Decomposition is not
quite essential to our definition, inasmuch as death may be followed by
drying-up[60], or by perpetual entombment in Siberian ice (as in the
well-known case of the mammoth), or by digestion in the stomach of a
beast of prey. But the notion of a dead body is indeed inseparably
connected with that of death, and I believe that I was right in
distinguishing between the division of an Infusorian into two
daughter-cells, and the death of a Metazoon, which leaves offspring
behind it, by calling attention to the absence of a dead body in the
process of fission among Infusoria (See below.). The real proof of
death is that the organized substance which previously gave rise to the
phenomena of life, for ever ceases to originate such phenomena. This,
and this alone, is what mankind has hitherto understood by death, and
we must start from this definition if we wish to retain a firm basis
for our considerations.

We must now consider whether this definition, derived from observation
of higher animals, may be also applied without alteration to the lower,
or whether the corresponding phenomena which arise in these latter,
differ in detail from those of the higher animals, so that a narrower
limitation of the above definition is rendered necessary.

Götte believes the process of encystment which takes place in so many
unicellular animals (Monoplastides) to be the analogue of death.
According to this authority, the individuals in question, not only
undergo a kind of winter sleep—a period of latent life—but when
surrounded by the cyst they lose their former specific organization;
they become a ‘homogeneous substance,’ and are resolved into a germ,
from which, by a process of development, a new individual of the same
species once more arises. The division of the contents of the cyst,
viz. its multiplication, is, according to this view, of secondary
importance, and the essential feature in the process is the
rejuvenescence of the individual. This rejuvenescence however is said
to not only consist in the simple transformation of the old individual,
but in its death, followed by the building up anew of another
individual. ‘The parent organism and its offspring are two successive
living stages of the same substance—separated, and at the same time
connected, by the condition of rejuvenescence which lies between them’
(l. c., p. 79). An ‘absolute continuity of life does not exist’; it is
only the dead organic matter which establishes the connection, and the
‘identity of this matter ensures heredity.’

It is certainly surprising that Götte should identify encystment with a
cessation of life, and we may well inquire for the evidence which is
believed to support such a view. The only evidence lies in a certain
degree of degeneration in the structure of the individual, and in the
cessation of the visible external phenomena of life, such as feeding
and moving. Does Götte really believe that it is an incorrect
interpretation of the facts to assume that a _vita minima_ continues to
exist in the protoplasm, after its complexity has diminished? Are we
compelled to invoke a mystical explanation of the facts, by an appeal
to such an indefinite principle as Götte’s rejuvenescence? Would not
the oxygen, dissolved in the water, affect the organic substance the
life of which it formerly maintained, and would it not cause its
decomposition, if it were in reality dead?

I, too, hold that the division of the encysted mass is of secondary
importance, and that the encystment itself, without the resulting
multiplication, is the original and essential part of the phenomenon.
But it does not follow from this that the encystment should be
considered as a process of rejuvenescence. What is there to be
rejuvenated? Certainly not the substance of the animal, for nothing is
added to it, and it can therefore acquire no new energy; and the forms
of energy which it manifests cannot be changed, since the form of the
matter is just the same after quitting the cyst as it was before.
Rejuvenescence has also been mentioned in connection with the process
of conjugation, but this is quite another thing. It is quite
reasonable, at least in a certain sense, to maintain the connection of
rejuvenescence with conjugation; for a fusion of the substance of two
individuals takes place, to a greater or lesser extent, in conjugation,
and the matter which composes each individual is therefore really
altered. But in simple encystment, rejuvenescence can only be
understood in the sense in which we speak of the fable of the Phœnix,
which, when old, was believed to be consumed by fire, and to rise again
from its own ashes as a young bird. I doubt whether this idea is in
agreement with the physiology of to-day, or with the laws of the
conservation of energy. It is easy to pull down an old house with
rotten beams and crumbling walls, but it would be impossible to build
it anew with the old material, even if we used new mortar, represented
in Götte’s hypothesis by water and oxygen. For these reasons I consider
the idea of rejuvenescence of the encysted individual to be contrary to
our present physiological knowledge.

It is much more simple and natural to regard encystment as adapted for
the protection of certain individuals in a colony from destruction by
being dried up or frozen, or for the protection of the individual
during multiplication by division, when it is helpless, and would
easily fall a prey to enemies, or to secure advantages in some other
way[61]. The case of _Actinosphaerium_, mentioned by Götte, clearly
demonstrates that rejuvenescence of the individual is not the only
event which happens during encystment, for this would scarcely require
six months. The long duration of latent life, from summer to the next
spring, clearly proves that encystment is of the highest importance for
the species, in order to maintain the life of the individual through
the dangers of an unfavourable season[62].

When in this case, the specific organization degenerates to a certain
extent, such changes depend in part upon the endeavour to diminish as
far as possible the size of the organism—the pseudopodia being drawn
in, while the vacuoles contract and completely disappear. The
degeneration may also, perhaps, depend in part upon the secretion of
the cyst itself, which implies a certain loss of substance[63]. But
degeneration chiefly depends upon the fact that the encystment is
accompanied by reproduction in the way of fission, which seems to begin
with a simplification of the organization, that is, with a fusion of
the numerous nuclei. It is well known that many unicellular animals
contain several nuclei—in other words, that the nuclear substance is
scattered in small parts throughout the whole cell. But when the animal
prepares for division, these pieces of nuclear substance fuse into a
single nucleus which itself undergoes division into two equal parts[64]
during the division of the animal. It is evident that the equal
division of the whole nuclear substance only becomes possible in this
way.

There are, however, numerous cases which prove that the bodies of
encysted animals may retain, during the whole process, exactly the same
structure and differentiation, which were previously characteristic of
them. Thus the large Infusorian _Tillina magna_, described by Gruber,
can be seen through the thin-walled cyst to retain the characteristic
structure of its ectoplasm, and the whole of its organization. Even the
movements of the enclosed animal do not cease; it continues to rotate
actively in the narrow cyst, as do the two or four parts into which it
subsequently divides. Such observations prove that Götte’s view that
‘every characteristic of the previous organization is lost,’ is quite
out of the question[65] (l. c., p. 62).

For this reason I must strongly oppose Götte’s view that an encysted
individual is a germ, viz. an organic mass still unorganized which can
only become an adult individual by means of a process of development. I
believe that an encysted individual is one possessing a protective
membrane, in structure more or less simplified as an adaptation to the
narrow space within the cyst, and to a possible subsequent increase by
division, in short one in which active life is reduced to a minimum,
and sometimes even completely in abeyance, as happens when it is frozen.

It is evident from the above considerations that encystment in no way
corresponds with that which every one, including myself, understands by
death, because during encystment one and the same being is first
apparently dead and then again alive; and we merely witness a condition
of rest, from which active life will again emerge. This would remain
true even if it were proved that life is, in reality, suspended for a
time. But such proof is still wanting, and Götte was apparently only
influenced by theoretical considerations, when he imagined that death
intervened where unprejudiced observers have only recognised a
condition of rest. He apparently entirely overlooked the fact that it
is possible to test his views; for all unicellular beings are in
reality capable of dying: we can kill them, for example, by boiling,
and they are then really dead and cannot be revived. But this state of
the organism differs chemically and physically from the encysted
condition, although we do not know all the details of the difference.
The encysted animal, when placed in fresh water, presently originates a
living individual, but the one killed by boiling only results in
decomposition of the dead organic matter. Hence we see that the same
external conditions give rise to different results in two different
states of the organism. It cannot be right to apply the same term to
two totally different states. There is only one phenomenon which can be
called death, although it may be produced by widely different causes.
But if the encysted condition is not identical with the death which we
can produce at will, then natural death, viz. that arising from
internal causes, does not exist at all among unicellular organisms.

These facts refute Götte’s peculiar view, which depends on the
existence of natural death among the Monoplastid organisms; upon proof
of the contradictory, his whole theory collapses. But there is
nevertheless a certain interest in following it further, for we shall
thus reach many ideas worthy of consideration.

First, the question arises as to how death could have been transmitted
from the Monoplastides[66] to the Polyplastides, a process which must
have taken place according to Götte. I will for the present omit the
fact that I cannot accept the supposition that the process of
encystment represents death. We may then inquire whether death has
taken the place of encystment among the Polyplastides, or, if this is
not the case, whether any process comparable to encystment exists among
the Polyplastides.

Götte believes that death is always connected with reproduction, and is
a consequence of the latter in both Protozoa and Metazoa. Reproduction
has, in his opinion, a directly ‘fatal effect,’ and the reproducing
individual must die. Thus the may-fly and the butterfly die directly
after laying their eggs, and the male bee dies immediately after
pairing; the Orthonectides expire after expelling their germ-cells,
while _Magosphaera_ resolves itself into germ-cells, and nothing
persists except these elements. It is but a step from this latter
organism to the unicellular animals which transform themselves as a
whole into germ-cells; but in order to achieve this they must undergo
the process of rejuvenescence, which Götte assumes to be the same as
death.

These views contain many fallacies quite apart from the soundness or
unsoundness of their foundation. The process of encystment, as Götte
thinks, represents, in the Monoplastides, true reproduction to which
multiplication by means of division has been secondarily added. This
encystment cannot be dispensed with, for internal causes determine that
it must occasionally interrupt the process of multiplication by simple
division. But, on the other hand, Götte also considers the division of
the contents of the cyst to be a secondary process. The essential
characteristic of encystment is a simple process of rejuvenescence
without multiplication. Hence we are forced to accept a primitive
condition in which simple division as well as the division of the
encysted individual were absent, and in which reproduction consisted
only in an often-repeated process of rejuvenescence among existing
individuals, without any increase in their number. Such a condition is
inconceivable because it would involve a rapid disappearance of the
species, and the whole consideration clearly shows us that division of
un-encysted individuals must have existed from the first, and that
this, and not a vague and mysterious rejuvenescence, has always been
the real and primitive reproduction of the Monoplastides. The fact that
encystment does not always lead to the division of the contents of the
cyst proves, in my opinion, that not reproduction but preservation
against injury from without, was the primitive meaning of encystment.
It is possible that at the present time there are but few Monoplastides
which are able to go through an infinite number of divisions without
the interposition of the resting condition implied by encystment;
although it has not yet been demonstrated for all species[67]. But it
is not right to conclude from this that there is an internal necessity
which leads to encystment, that is to say to identify this process with
rejuvenescence. It is much more probable that encystment is merely an
adaptation to continual changes in the external conditions of life,
such as drought and frost, and perhaps also the want of food which
arises from the over-population of small areas. The same phenomenon is
known in certain low Crustacea—the _Daphnidae_—which possess an
ephippium or protective case for their winter-eggs. This case is only
developed after a certain definite number of generations has been run
through, an event which may happen at any time in the year in species
living in pools which are liable to be often dried-up; but only in the
autumn in such as live in lakes which are never dry. No one ever
doubted that the periodical formation of the ephippium in certain
generations was an adaptation to changes in the external conditions of
life.

Even if the process of rejuvenescence in the Monoplastides were really
equivalent to the death of the higher animals, we could not conclude
from this that it is necessarily associated with reproduction.
Encystment alone is not reproduction, and it first becomes a form of
reproduction when it is associated with the division of the encysted
animal. Simple division was the true and original form of reproduction
in Monoplastides, and even now it is the principal and fundamental form.

Hence we see that among the Monoplastides reproduction is not connected
with death, even if we accept Götte’s view and allow that encystment
represents death. I shall return later on to the relation between death
and reproduction in the Metazoa; but the question first arises whether
encystment, if it is not death, has any analogue in the higher animals,
and further whether death takes that place in their development which
is occupied by encystment in the Monoplastides.

Among the higher Metazoa there can be no doubt as to what we mean by
death, but the precise nature of that which dies is not equally
evident, and the popular conception is not sufficient for us. It is
necessary to distinguish between the mortal and the immortal part of
the individual—the body in its narrower sense (_soma_) and the
germ-cells. Death only affects the former; the germ-cells are
potentially immortal, in so far as they are able, under favourable
circumstances, to develope into a new individual, or, in other words,
to surround themselves with a new body (_soma_)[68].

But how is it with the lowest Polyplastides in which there is no
antithesis between the somatic and germ-cells, and among which each of
the component cells of the multicellular body has retained all the
animal functions of the Monoplastides, even including reproduction?

Götte believes that the natural death of these organisms (which he
rightly calls Homoplastides) consists in ‘the dissolution of the
cell-colony.’ As an example of such dissolution Götte takes Häckel’s
_Magosphaera planula_, a marine free-swimming organism in the form of a
sphere composed of a single layer of ciliated cells, imbedded in a
jelly. (For figure see below.) This organism cannot however be
‘considered as a genuine perfect Polyplastid, for at a certain time the
component cells part from one another and then continue to live
independently in the condition of Monoplastides.’ These free amoebiform
organisms increase considerably in size, encyst, and finally undergo
numerous divisions—a kind of segmentation within the cyst. The result
of the division is a sphere of ciliated cells similar to that with
which the cycle began. In fact, _Magosphaera_ is not a perfect
Polyplastid, but a transitional form between Polyplastides and
Monoplastides, as the discoverer of the group of animals of which it is
the only representative, indicated, when he named the group
‘Catallacta.’

  [Illustration: Development of Magosphaera Planula (after Häckel).

  1. Encysted amoeboid form. 2 and 3. Two stages in the division of the
  same. 4. Free ciliated sphere, the cells of which are connected by a
  gelatinous mass. 5. One of the ciliated cells which has become free
  by the breaking up of the sphere. 6. The same in the amoeboid form.
  7. The same grown to a larger size.]

According to Götte, the natural death of _Magosphaera_ consists, as in
the undoubted Protozoa, in a process of rejuvenescence by encystment.
The dissolution of the ciliated sphere into single cells ‘cannot be
identical with natural death. For the regular and complete separation
of the _Magosphaera_-cells proves that their individuality has not been
completely subordinated to that of the whole colony, and it proves that
the latter is not completely individualised[69].’

Nothing can be said against this, if we agree in identifying death with
the encystment of the Monoplastides. Now we could, as Götte rightly
remarks, derive the lower forms of Polyplastides from _Magosphaera_ if
‘the connection between the cells of the ciliated sphere were retained
until encystment, viz. until the reproduction of the single cells had
taken place[70].’ After this had been accomplished, Götte considers
that death would consist ‘in the complete separation of the cells from
one another, accompanied in all probability by their simultaneous
change into germ-cells.’ The fallacy in this is evident; if death is
represented in one case by the encystment during which single cells
change into germ-cells, then this must apply to the other case also,
for nothing has changed except the duration of the cell-colony. The
nature of encystment cannot be affected by the fact that the cells
separate from one another a little earlier or a little later. If it is
true that death is represented by encystment among the Monoplastides,
then the same conclusion must also hold for the Polyplastides; or
rather death must be represented in them by the process of
rejuvenescence, which Götte considers to be the essential part of
encystment. Götte ought not to identify death with the dissolution of
the cell-colony of which the lowest and highest Polyplastides are alike
composed; but he should seek it in the process of rejuvenescence which
takes place within the germ-cells. If it is essential to the nature of
reproduction that the cells set apart for that purpose should pass
through a process of rejuvenescence, which is equivalent to death, then
this must be true for the reproductive cells of all organisms. If these
conclusions hold good, there is nothing to prevent us from assuming
that such a process of rejuvenescence actually occurs in the higher
animals. Götte evidently holds this view, as is plainly shown in the
last pages of his essay. He there attempts to bring his views of the
death and rejuvenescence of the germ into harmony with his previously
developed idea of the derivation of death among the Polyplastides from
the dissolution of the cell-colonies. Götte still clings to the view
which he propounded in describing the development of _Bombinator_,
according to which the egg-cell of the higher Metazoa must pass through
a process of rejuvenescence representing death, before it can become a
germ.

According to Götte’s[71] idea ‘the egg of a _Bombinator igneus_ before
fertilization cannot be considered to be a cell either wholly or in
part; and this is equally true of it at its origin and after its
complete development; it is only an essentially homogeneous organic
mass enclosed by a membrane which has been deposited externally.’ This
mass is ‘unorganised and not living[72],’ and ‘during the first
phenomena of its development all vital powers must be excluded.’ In
this way the continuity of life between two successive individuals is
always interrupted; or, as Götte says in his last essay:—‘The
continuity of life between individuals of which one is derived from the
other by means of reproduction, exists neither in the rejuvenescence of
the Monoplastides nor in the condition of the germ among the
Polyplastides—a condition which is derived from the former[73].’

This is quite logical, although in my opinion it is both unproved and
incorrect. But, on the other hand, it is certainly illogical for Götte
to derive the death of the Metazoa in a totally different way, i. e.
from the dissolution of their cell-colonies. It is quite plain that the
death of the Metazoa does not especially concern the reproductive
cells, but the individual which bears them; Götte must therefore seek
for some other origin of death—an origin which will enable it to reach
the body (_soma_)—as opposed to the germ-cells. If there still remained
any doubt about the failure to establish a correspondence between death
and the encystment of the Monoplastides, we have here, at any rate, a
final demonstration of the failure!

But there is yet another great fallacy concealed in this derivation of
the death of the Polyplastides.

Among the lowest Polyplastides, where all the cells still remain
similar, and where each cell is also a reproductive cell, the
dissolution of the cell-colony is, according to Götte, to be regarded
as death, inasmuch as ‘the integrity of the mother-individual
absolutely comes to an end’ (l. c., p. 78). The dissolution of a
cell-colony into its component living elements can only be called death
in the most figurative sense, and can have nothing to do with the real
death of the individuals; it only consists in a change from a higher to
a lower stage of individuality. Could we not kill a _Magosphaera_ by
boiling or by some other artificial means, and would not the state
which followed be death? Even if we define death as an arrest of life,
the dissolution of _Magosphaera_ into many single cells which still
live, is not death, for life does not cease in the organic matter of
which the sphere was composed, but expresses itself in another form. It
is mere sophistry to say that life ceases because the cells are no
longer combined into a colony. Life does not in truth cease for a
moment. Nothing concrete dies in the dissolution of _Magosphaera_;
there is no death of a cell-colony, but only of a conception. The
Homoplastides, that is cell-colonies built up of equal cells, have not
yet gained any natural death, because each of their cells is, at the
same time, a somatic as well as a reproductive cell: and they cannot be
subject to natural death, or the species would become extinct.

It is more to the purpose that Götte has sought for an illustration of
death among those remarkable parasites, the Orthonectides, because in
them we do at any rate meet with real death. They are indeed very low
organisms; but nevertheless they stand far above _Magosphaera_, even if
the latter were hypothetically perfected up to the level of a true
Homoplastid, for the cells which compose the body of the Orthonectides
are not all similar, but are so far differentiated that they are even
arranged in the primitive germ-layers, and a form results which has
rightly been compared with that of the Gastrula. It is true they are
not quite so simple as Götte[74] figures them, for they not only
consist of ectoderm and germ-cells, but, according to Julin[75], the
endoderm is arranged in two layers—the germ-cells and a layer which
forms during development a strong muscular coat; and in the second
female form the egg-cells are surrounded by a tolerably thick granular
tissue.

  [Illustration: Orthonectides (after Julin).

  8. First female form: the cap-like anterior part has become detached
  and the egg-cells (_eiz_) are escaping. 9. Second female form: _eiz_
  = egg-cells; outside these are the muscular layer (_m_) and the
  ectoderm (_ekt_). 10 and 11. Two fragments of such a female broken to
  pieces by spontaneous division: the egg-cells are embedded in a
  granular mass, and undergo embryonic development in it at a later
  period; the whole is surrounded by ciliated cells. 12. Male
  discharging the spermatozoa by the breaking up of the ectoderm
  (_ekt_); _sp_ spermatozoa; _m_ muscle.]

There is nevertheless no doubt that in the first female form, when
sexually mature, the greater part, not only of the endoderm but of the
whole body, is made up of ova, so that the animal resembles a
thin-walled sac full of eggs. The ova escape by the bursting of the
thin ectoderm, and when they have all escaped, the thin disintegrated
membrane, composed of ciliated cells, is no longer in a condition to
live, and dies at once. This is the course of events as described by
Götte, and he is probably correct in his interpretation. This is the
real death of the Orthonectides, and if we regard them as low primitive
forms (Mesozoa), here for the first time in the ascending series we
meet with natural death. But the causes of this are scarcely so clear
as Götte seems to think when he ascribes it to the effect of
reproduction—an effect which is ‘not only empirically necessary, but
absolutely unavoidable.’ Such a necessity is explained by the fact that
the endoderm consists entirely of germ-cells. Now the life of the
organism, being dependent upon the mutual action of both layers, must
cease as soon as the whole endoderm is extruded during reproduction.

Arguments such as these pass over the presence of a mesoderm; but apart
from this omission, it does not appear to me so self-evident from a
purely physiological standpoint, that the ectodermal sheath with its
muscle layer must die after the extrusion of the germ-cells.

In those females to which Götte refers in this passage, the whole
sheath remains at first quite uninjured, with the exception of a small
cap at the anterior end, which is pushed off to give exit to the ova;
and inasmuch as the sheath continues to swim about in the nutritive
fluids after this has taken place, the proof is at any rate wanting
that it cannot support itself quite as well as before, although it has
lost the germ-cells.

Then why does it die? My answer to this is simple:—because it has lived
its time; because its length of life is limited to a period which
corresponds with the time necessary for complete reproduction. The
physical constitution of the body is so regulated that it remains
capable of living until the extrusion of the reproductive cells, and
then dies, however favourable external conditions may be for its
further support.

The correctness of this explanation is shown by a consideration of the
males and the second form of females; for in these cases the body falls
to pieces, not as a consequence of reproduction, but as a preparation
for it!

Götte only mentions the second female form in a note, in which he says,
it appears ‘that in the second female form of these animals the whole
body breaks into many pieces, and the superficial layer gradually
atrophies, so that it dies before the eggs are extruded.’ In Julin’s
account[76], upon which Götte bases his statements, there are, however,
some not unimportant differences. For instance, the eggs are not
extruded at all, but embryonic development takes place within the body
of the mother, which has previously undergone spontaneous division into
several pieces. In this case, the eggs differ from those of the other
female form, inasmuch as they do not constitute the whole of the
endoderm, but are embedded (as was stated above) in a fairly voluminous
granular mass at the expense of which, or at least by means of which,
they are nourished; for they increase considerably in size during their
development. But not only this granular mass, but all the layers of the
body of the mother, even the ectoderm, persist during the embryonic
development of the offspring. Indeed, the ectoderm must continue to
grow during the division of the mother animal, for it gradually covers
in the products of division on all sides, and, by means of its cilia,
causes the animal to swim about in the fluids of its host. After some
time the cilia are lost, and the separate parts into which the mother
animal has divided, fix themselves upon some part of the body-cavity of
the host; the young become free, and the remains of the body of the
mother probably disappear by dissolution and resorption[77]. In this
case the remains of the mother animal seem to be, to some extent,
consumed by the embryos,—a process which sometimes, although very
rarely, happens elsewhere. We can scarcely consider this as a primitive
arrangement, or look upon it as a proof that ‘reproduction’ has a
necessarily fatal effect upon the Polyplastid organism.

In the male, the mass of spermatozoa does not swell out the body to
such an extent that its walls must give way and thus permit an exit,
but the large ectoderm cells atrophy spontaneously at the time of
maturity, and as they fall off, exit is given to the spermatozoa here
and there. In this instance also the dissolution of the body is not a
consequence of reproduction, but reproduction can only take place when
the dissolution of the body has preceded it!

In this remarkable arrangement we cannot discern anything except an
evident adaptation of the life of the body-cells to reproductive
purposes, and this adaptation was rendered possible because, after the
evacuation of the sexual cells, the body ceased to be of any value for
the maintenance of the species.

But even if we assume, that the death of the Orthonectides is, in
Götte’s sense, a consequence of reproduction, inasmuch as, in the two
forms of females as well as in the male, the extrusion of a mass of
developed germ-cells or embryos deprives the organism of the
physiological possibility of living longer, how can we explain the
necessity of death as a direct consequence of reproduction in all
Polyplastides? Is the body—the _soma_—of the Metazoa so imperfectly
developed, as compared with the reproductive cells, that the extrusion
of the latter involves the death of the former? As a matter of fact in
the majority of cases the relations are reversed; the number of
body-cells usually exceeds the germ-cells a hundred- or a
thousand-fold, and the body is, as regards nutrition, so completely
independent of the reproductive cells, that it need not be in the least
disadvantageously affected by their extrusion. And if the
Orthonectid-like ancestors of the Metazoa were compelled to give up
their insignificant somatic part after the extrusion of their
germ-cells, because it could now no longer support itself, does it
therefore follow that the somatic cells had for ever lost the power of
surviving, even when their phyletic descendants were surrounded by more
favourable conditions? Had they to inherit ‘the necessity of death’ for
all time? Whence came this great change in the nature of organisms
which, before the differentiation of Homoplastids into Heteroplastids,
were endowed with the immortality of unicellular beings?

And it must be remembered that it is only an assumption which places
the Orthonectides among the lowest Metazoa (Heteroplastids). I do not
intend to greatly emphasize this point, but the formation of the
Gastrula by embole, and the absence of a mouth and alimentary canal,
shows that these parasites are extremely degenerate, and the same may
be said of almost all endoparasites. The Gastrula, as an independent
organism, was without doubt primitively provided with both mouth and
stomach, and the mass of ova filling the female Orthonectid is an
adaptation to a parasitic life, which on the one side renders the
possession of a stomach a superfluity, and on the other demands the
production of a great number of germ-cells[78]. It is certain that the
Orthonectides, as at present constituted, cannot have lived in the free
condition, and also that their adaptation to parasitism cannot have
arisen at the beginning of the phyletic development of Metazoa, because
they inhabit star-fishes and Nemertines—both relatively highly
developed Metazoa. Hence it is, at any rate, doubtful whether the
Orthonectides can claim to pass as typical forms of the lowest
Heteroplastids, and whether their reproduction can be looked upon ‘as
typical for the unknown ancestors of all Polyplastids’ (l. c., p. 45).
If, however, we accept some organism resembling these Orthonectides as
the most ancient Heteroplastid, being a free-living organism, it must
have had a stomach, and the cells surrounding it must—as a whole or in
part—have possessed the power of digesting; at any rate, they cannot
all have been germ-cells, and therefore it is improbable that death
would be the direct result of the extrusion of the germ-cells.

Let us now consider the manner in which Götte has endeavoured to
explain the transmission of the cause of death—which first appeared in
the Orthonectides—from these organisms to all later Metazoa, until the
very highest forms are reached. Exact proofs of this supposition are
unfortunately wanting, and the evidence is confined to the collection
of a number of cases in which death and reproduction take place nearly
or quite simultaneously. These would prove nothing, even if _post hoc_
were always _propter hoc_; and there are, opposed to them, a number of
cases in which reproduction and death take place at different times. In
obtaining evidence for ‘the fatal influence of reproduction,’ is it
possible to point to every case of sudden death after the act of
oviposition or fertilization? These cases occur among many of the
higher animals, especially in Insects, and were collected by me in an
earlier work[79]. It is obvious that such cases are exceptional, but in
a restricted sense it is quite true, as far as these individual
instances are concerned, that death appears as a consequence of
reproduction. The male bee, which invariably dies while pairing, is
undoubtedly killed in consequence of a very powerful nervous shock; and
the female Psychid, which has laid all her eggs at once, dies of
‘exhaustion’—however we may attempt to explain the term on
physiological principles.

Can we conclude from these cases that the effects of reproduction are,
in Götte’s sense, universally fatal; that reproduction is the positive
and ‘exclusive explanation of natural death’? (l. c., p. 32.) I need
not linger over these isolated examples, but I turn at once to the
foundation of the whole conclusion—a foundation which is obviously
unable to support the superstructure erected on it. Götte formally
derives the idea that death is a necessary condition of reproduction,
from a very heterogeneous collection of facts. When we examine this
collection we find that the process which is taken to be death is not
the same thing in all these instances, while the same is true of the
influence of reproduction by which death is supposed to be caused. The
whole conception arises out of the process of encystment, which is
regarded as the building-up of reproductive material—that is, as true
reproduction; and since, according to Götte’s view, the formation of
germs is always intimately connected with an arrest of life, and since,
by his own definition, this stand-still of life is equivalent to death,
it follows that, with such a theory, reproduction, in its essential
nature, must be inseparably connected with death. It is necessary at
this juncture to remember what Götte means by the process of
rejuvenescence, and to point out that he is dealing with something
quite different from ‘the fatal influence of reproduction,’ which was
just now mentioned with regard to insects. ‘Rejuvenescence,’ bound up
as it is with encystment and reproduction, is, according to Götte, ‘a
re-coining of the specific protoplasm, by means of which the identity
of its substance is fixed by heredity,’ a ‘marvellous process in which
phenomena the most important in the whole life of the animal, and in
fact of all organisms—reproduction and death—have their roots’ (l. c.,
p. 81). Whether such re-coining really takes place or not, at any rate
I claim to have shown above that it does not correspond with death in
the Metazoa, and—if it is represented at all in these latter—that it
ought to be looked for in the reproductive cells; and indeed, in
another passage, Götte himself has placed the process in these cells.

While, among the Monoplastids, according to Götte, the causes of the
supposed death lie hidden in this mysterious change of the organism
into reproductive material, Götte asserts that among the Polyplastids
(such as _Magosphaera_, hypothetically perfected so as to form a
genuine Polyplastid), the causes of death operate so that the organism
breaks up into its component cells, all these being still reproductive
cells—a process which, unlike ‘rejuvenescence,’ has nothing mysterious
about it, and which is certainly not genuine death. In the
Orthonectid-like animals death does not occur as a consequence of the
dispersal of the reproductive cells, but rather because the part of the
animal which remains is so small and effete that, being unable to
support itself, it necessarily dies. From this point at least the
object of death and the conception of it remain the same, but now the
idea of reproduction undergoes a change. When the Rhabdite females of
_Ascaris_ are eaten up by their offspring, is this mode of death
connected with the ‘rejuvenescence of protoplasm’? (l. c., p. 34.) Is
there any deep underlying relationship between such an end and the
essential nature of reproduction? The same question may be asked with
regard to the ‘Redia or the Sporocyst of Trematodes, which are
converted into slowly dying sacs during the growth of the Cercariae
within them.’ We cannot speak of the ‘fatal influence of reproduction’
among tape-worms just because ‘in the ripe segments the whole
organization degenerates under the influence of the excessive growth of
the uterus.’ It certainly degenerates, but only so far as the
developing mass of eggs demands. In fact, at a sufficiently high
temperature, death does not occur, and such mature segments of
tape-worms creep about of their own accord. We cannot fail to recognize
that in this as well as in the above-mentioned cases we have to do with
adaptation to certain very special conditions of existence—an
adaptation leading to an immense development of reproductive cells in a
mother organism which can no longer take in nourishment, or which has
become entirely superfluous because its duty to its species is already
fulfilled. If this is an example of death inherent in the essential
nature of reproduction, then so is the death of a mature segment of a
tapeworm in the gastric juices of the pig that eats it.

With Götte, the conception of reproduction, like the conception of
death, is a protean form, which he welcomes in any shape, if only he
can use it as evidence. If death is a necessary consequence of
reproduction, its cause must be always essentially the same, and might
be expressed in one of the following suggestions:—(1) in the necessity
for a ‘re-coining’ of the protoplasm of the germ-cells; but here death
could only affect the germ-cells themselves: (2) perhaps in the
withdrawal of nourishment by the mass of developing reproductive
material, just as death occurs sometimes among men by the excessive
drain on the system caused by morbid tumours: (3) or in consequence of
the development of the offspring in the body of the mother; this
however would only affect the females, and could therefore have no deep
and general significance: (4) from the extrusion of the sexual
cells,—ova or spermatozoa,—and in the impossibility of further
nourishment which is consequent upon this extrusion—(Orthonectides?):
or (5) finally in an excessively powerful nervous shock brought about
by the ejection of the reproductive cells.

But no one of these alternatives is the universal and inevitable cause
of death. This proves irrefutably that death does not proceed as an
intrinsic necessity from reproduction, although it may be connected
with the latter, sometimes in one way and sometimes in another. But we
must not overlook the fact that in many cases death is not connected
with reproduction at all; for many Metazoa survive for a longer or
shorter period after the reproductive processes have ceased.

In fact, I believe I have definitely shown that no process exists among
unicellular animals which is at all comparable with the natural death
of the higher organisms. Natural death first appeared among
multicellular beings, and among these first in the Heteroplastids.
Furthermore, it was not introduced from any absolute intrinsic
necessity inherent in the nature of living matter, but on grounds of
utility, that is from necessities which sprang up, not from the general
conditions of life, but from those special conditions which dominate
the life of multicellular organisms. If this were not so, unicellular
beings must also have been endowed with natural death. I have already
expressed these ideas elsewhere[80], and have briefly indicated how
far, in my opinion, natural death is expedient for multicellular
organisms. I found the essential reason for confining the life of the
Metazoa to a fixed and limited period, in the wear and tear to which an
individual is exposed in the course of a life-time. For this reason,
‘the longer the individual lived, the more defective and crippled it
would become, and the less perfectly would it fulfil the purpose of its
species’ (l. c., p. 24). Death seemed to me to be expedient since
‘worn-out individuals are not only valueless to the species, but they
are even harmful, for they take the place of those which are sound’ (l.
c., p. 24).

I still adhere entirely to this explanation; not of course in the sense
that an actual physical struggle has ever taken place between the
mortal and immortal varieties of any species. If Götte understood me
thus, he may be justified by the brief explanations given in the essay
to which I have alluded; but when he also attributes to me the opinion
that such hypothetically immortal Metazoa had but a very limited period
for reproduction, I fail to see what part of the essay in question can
be brought forward in support of his statement. Only under some such
supposition can I be reproached with having assumed the existence of a
process of natural selection which could never be effective, because
any advantage which accrued to the species from the shortening of the
duration of life could not make itself felt in a more rapid propagation
of the short-lived individuals. The statement ‘that in this and in
every other case it is a sufficient explanation of the processes of
natural selection to render it probable that any kind of advantage is
gained’[81] is indeed erroneous. The explanation ought rather to be
‘that the forms in question would for ever transmit their characters to
a greater number of descendants than the other forms.’ I have not
however as yet attempted to think out in detail such processes of
natural selection as would limit the somatic part of the Metazoan body
to a short term of existence, and I only wished to emphasize the
general principle lying at the basis of the whole process, without
stating the precise manner in which it operates.

If I now attempt to take this course, and to reconstruct theoretically
the gradual appearance of natural death in the Metazoa, I must begin by
again alluding to Götte’s criticisms in reference to the operation of
natural selection.

I consider death as an adaptation, and believe that it has arisen by
the operation of natural selection. Götte[82], however, concludes from
this that ‘the first origin of hereditary and consequently (for the
organization in question) necessary death, is not explained but already
assumed.’ ‘The operation and significance of the principle of utility
consists in selecting the fittest from among the structures and
processes which are at hand, and not in directly creating new ones.
Every new structure arises at first, quite independently of any
utility, from certain material causes present in a number of
individuals, and when it has proved useful and is transmitted, it
extends, according to the laws of the survival of the fittest, in the
group of animals in which it appeared. This extension will undergo
further increase with every advance in utility which results from
further structural changes, until it extends over the whole group. So
that usefulness effects the preservation and the distribution of new
structures, but has nothing whatever to do with the causes of their
primary origin and their consequent transmission to all other
individuals. Indeed, on these hereditary causes the necessity of the
structures in question depends, so that their usefulness in no way
explains their necessity.’

‘These conclusions, when applied to the origin of natural death called
forth by internal causes, would show that it became inevitable and
hereditary in a number of the originally immortal Metazoa, before there
could be any question as to the benefits derived from its influence.
Such influence must have consisted in the fact that more descendants
survived the struggle for existence and were able to enter upon
reproduction among the individuals which had inherited the
predisposition to die than among the potentially immortal beings which
would be damaged in the struggle for existence, and would therefore be
exposed to still further injuries. The existing necessity for natural
death in all Metazoa might therefore be derived in an unbroken line of
descent from the first mortal Metazoan, of which the death became
inevitable from internal causes, before the principle of utility could
operate in favour of its dissemination.’

In reply to this I would urge: that it has been very often maintained
that natural selection can produce nothing new, but can only bring to
the front something which existed previously to the exercise of choice;
but this argument is only true in a very limited sense. The complex
world of plants and animals which we see around us contains much that
we should call new in comparison with the primitive beings from which,
as we believe, everything has developed by means of natural selection.
No leaves or flowers, no digestive system, no lungs, legs, wings, bones
or muscles were present in the primitive forms, and all these must have
arisen from them according to the principle of natural selection. These
primitive forms were in a certain sense predestined to develope them,
but only as possibilities, and not of necessity; nor were they
preformed in them. The course of development, as it actually took
place, first became a necessity by the action of natural selection,
that is by the choice of various possibilities, according to their
usefulness in fitting the organism for its external conditions of life.
If we once accept the principle of natural selection, then we must
admit that it really can create new structures, instincts, etc., not
suddenly or discontinuously, but working by the smallest stages upon
the variations that appear. These changes or variations must be looked
upon as very insignificant, and are, as I have of late attempted to
show[83], quantitative in their nature; and it is only by their
accumulation that changes arise which are sufficiently striking to
attract our attention, so that we call them ‘new’ organs, instincts,
etc.

These processes may be compared to a man on a journey who proceeds from
a certain point on foot by short stages, at any given time, and in any
direction. He has then the choice of an infinite number of routes over
the whole earth. If such a man begins his wanderings in obedience to
the impulse of his own will, his own pleasure or interest,—proceeding
forwards, to the right or left, or even backwards, with longer or
shorter pauses, and starting at any particular time,—it is obvious that
the route taken lies in the man himself and is determined by his own
peculiar temperament. His judgment, experience, and inclination will
influence his course at each turn of his journey, as new circumstances
arise. He will turn aside from a mountain which he considers too lofty
to be climbed; he will incline to the right, if this direction appears
to afford a better passage over a swollen stream; he will rest when he
reaches a pleasant halting-place, and will hurry on when he knows that
enemies beset him. And in spite of the perfectly free choice open to
him, the course he takes is in fact decided by both the place and time
of his starting and by circumstances which—always occurring at every
part of the journey—impel him one way or the other; and if all the
factors could be ascertained in the minutest detail, his course could
be predicted from the beginning.

Such a traveller represents a species, and his route corresponds with
the changes which are induced in it by natural selection. The changes
are determined by the physical nature of the species, and by the
conditions of life by which it is surrounded at any given time. A
number of different changes may occur at every point, but only that one
will actually develope which is the most useful, under existing
external conditions. The species will remain unaltered as long as it is
in perfect equilibrium with its surroundings, and as soon as this
equilibrium is disturbed it will commence to change. It may also happen
that, in spite of all the pressure of competing species, no further
change occurs because no one of the innumerable very slight changes,
which are alone possible at any one time, can help in the struggle;
just as the traveller who is followed by an overpowering enemy, is
compelled to succumb when he has been driven down to the sea. A boat
alone could save him, without it he must perish; and so it sometimes
happens that a species can only be saved from destruction by changes of
a conspicuous kind, and these it is unable to produce.

And just as the traveller, in the course of his life, can wander an
unlimited distance from his starting-point, and may take the most
tortuous and winding route, so the structure of the original organism
has undergone manifold changes during its terrestrial life. And just as
the traveller at first doubts whether he will ever get beyond the
immediate neighbourhood of his starting-point, and yet after some years
finds himself very far removed from it—so the insignificant changes
which distinguish the first set of generations of an organism lead on
through innumerable other sets, to forms which seem totally different
from the first, but which have descended from them by the most gradual
transition. All this is so obvious that there is hardly any need of a
metaphor to explain it, and yet it is frequently misunderstood, as
shown by the assertion that natural selection can create nothing new:
the fact being that it so adds up and combines the insignificant small
deviations presented by natural variation, that it is continually
producing something new.

If we consider the introduction of natural death in connection with the
foregoing statements, we may imagine the process as taking place in
such a way that,—with the differentiation of Heteroplastids from
Homoplastids, and the appearance of division of labour among the
homogeneous cell-colonies,—natural selection not only operated upon the
physiological peculiarities of feeding, moving, feeling, or
reproduction, but also upon the duration of the life of single cells.
At this developmental stage there would, at any rate, be no further
necessity for maintaining the power of limitless duration. The somatic
cells might therefore assume a constitution which excluded the
possibility of unending life, provided only that such a constitution
was advantageous for them.

It may be objected that cells of which the ancestors possessed the
power of living for ever, could not become potentially mortal (that is
subject to death from internal causes) either suddenly or gradually,
for such a change would contradict the supposition which attributes
immortality to their ancestors and to the products of their division.
This argument is valid, but it only applies so long as the descendants
retain the original constitution. But as soon as the two products of
the fission of a potentially immortal cell acquire different
constitutions by unequal fission, another possibility arises. Now it is
conceivable that one of the products of fission might preserve the
physical constitution necessary for immortality, but not the other;
just as it is conceivable that such a cell—adapted for unending
life—might bud off a small part, which would live a long time without
the full capabilities of life possessed by the parent cell; again, it
is possible that such a cell might extrude a certain amount of organic
matter (a true excretion) which is already dead at the moment it leaves
the body. Thus it is possible that true unequal cell-division, in which
only one half possesses the condition necessary for increasing, may
take place; and in the same way it is conceivable that the constitution
of a cell determines the fixed duration of its life, examples of which
are before us in the great number of cells in the higher Metazoa, which
are destroyed by their functions. The more specialized a cell becomes,
or in other words, the more it is intrusted with only one distinct
function, the more likely is this to be the case: who then can tell us,
whether the limited duration of life was brought about in consequence
of the restricted functions of the cell or whether it was determined by
other advantages[84]? In either case we must maintain that the
disadvantages arising from a limited duration of the cells are more
than compensated for by the advantages which result from their highly
effective specialized functions. Although no one of the functions of
the body is necessarily attended by the limited duration of the cells
which perform it, as is proved by the persistence of unicellular forms,
yet any or all of them might lead to such a limitation of existence
without in any way injuring the species, as is proved by the Metazoa.
But the reproductive cells cannot be limited in this way, and they
alone are free from it. They could not lose their immortality, if
indeed the Metazoa are derived from the immortal Protozoa, for from the
very nature of that immortality it cannot be lost. From this point of
view the body, or _soma_, appears in a certain sense as a secondary
appendage of the real bearer of life,—the reproductive cells.

Just as it was possible for the specific somatic cells to be
differentiated from among the chemico-physical variations which
presented themselves in the protoplasm, by means of natural selection,
until finally each function of the body was performed by its own
special kind of cell; so it might be possible for only those variations
to persist the constitution of which involved a cessation of activity
after a certain fixed time. If this became true of the whole mass of
somatic cells, we should then meet with natural death for the first
time. Whether we ought to regard this limitation of the life of the
specific somatic cells as a mere consequence of their differentiation,
or at the same time as a consequence of the powers of natural selection
especially directed to such an end,—appears doubtful. But I am myself
rather inclined to take the latter view, for if it was advantageous to
the somatic cells to preserve the unending life of their ancestors—the
unicellular organisms, this end might have been achieved, just as it
was possible at a later period, in the higher Metazoa, to prolong both
the duration of life and of reproduction a hundred- or a thousand-fold.
At any rate, no reason can be given which would demonstrate the
impossibility of such an achievement.

With our inadequate knowledge it is difficult to surmise the immediate
causes of such a selective process. Who can point out with any feeling
of confidence, the direct advantages in which somatic cells, capable of
limited duration, excelled those capable of eternal duration? Perhaps
it was in a better performance of their special physiological tasks,
perhaps in additional material and energy available for the
reproductive cells as a result of this renunciation of the somatic
cells; or perhaps such additional power conferred upon the whole
organism a greater power of resistance in the struggle for existence,
than it would have had, if it had been necessary to regulate all the
cells to a corresponding duration.

But we are not at present able to obtain a clear conception of the
internal conditions of the organism, especially when we are dealing
with the lowest Metazoa, which seem to be very rarely found at the
present day, and of which the vital phenomena we only know as they are
exhibited by two species, both of doubtful origin. Both species have
furthermore lost much of their original nature, both in structure and
function, as a result of their parasitic mode of life. Of the
Orthonectides and Dicyemidae we know something, but of the reproduction
in the single free non-parasitic form, discovered by F. E. Schulze and
named by him _Trichoplax adhaerens_, we know nothing whatever, and of
its vital phenomena too little to be of any value for the purpose of
this essay.

At this point it is advisable to return once more to the derivation of
death in the Metazoa from the Orthonectides, as Götte endeavoured to
derive it, when he overlooked the fact that, according to his theory,
natural death is inherited from the Monoplastids and cannot therefore
have arisen anew in the Polyplastids. According to this theory, death
must necessarily have appeared in the lowest Metazoa as a result of the
extrusion of the germ-cells, and by continual repetition must have
become hereditary. We must not however forget that, in this case, the
cause of death is exclusively external, by which I mean that the
somatic cells which remained after the extrusion of the reproductive
cells, were unable to feed any longer or at any rate to an adequate
extent; and that the cause of their death did not lie in their
constitution, but in the unfavourable conditions which surrounded them.
This is not so much a process of natural death as of artificial death,
regularly appearing in each individual at a corresponding period,
because, at a certain time of life, the organism becomes influenced by
the same unfavourable conditions. It is just as if the conditions of
life invariably led to death by starvation at a certain stage in the
life of a certain species. But we know that death arises from purely
internal causes among the higher Metazoa, and that it is anticipated by
the whole organisation as the normal end of life. Hence nothing is
gained by this explanation founded on the Orthonectides, and we should
have to seek further and in a later stage of the development of the
Metazoa, for the internal causes of true natural death.

Another theory might be based upon the supposition that natural death
has been derived, in the course of time, from an artificial death which
always appeared at the same stage of each individual life—as we have
supposed to be the case in the Orthonectides. I cannot agree with this
view, because it involves the transmission of acquired characters,
which is at present unproved and must not be assumed to occur until it
has been either directly or indirectly demonstrated[85]. I cannot
imagine any way in which the somatic cells could communicate this
assumed death by starvation to the reproductive cells in such a manner
that the somatic cells of the resulting offspring would spontaneously
die of hunger in the same manner and at a corresponding time as those
of the parent. It would be as impossible to imagine a theoretical
conception of such transmission as of the supposed instance of kittens
being born without a tail after the parent’s tail had been docked;
although to make the cases parallel the kittens’ tails ought to be lost
at the same period of life as that at which the parent lost hers. And
in my opinion we do not add to the intelligibility of such an idea by
assuming the artificial removal of tails through hundreds of
generations. Such changes, and indeed all changes, are, as I think,
only conceivable and indeed possible when they arise from within, that
is, when they arise from changes in the reproductive cells. But I find
no difficulty in believing that variations in these cells took place
during the transition from Homoplastids to Heteroplastids, variations
which formed the material upon which the unceasing process of natural
selection could operate, and thus led to the differentiation of the
previously identical cells of the colony into dissimilar ones—some
becoming perishable somatic cells, and others the immortal reproductive
cells.

It is at any rate a delusion to believe that we have explained natural
death, by deriving it from the starvation of the _soma_ of the
Orthonectides, by the aid of the unproved assumption of the
transmission of acquired variations. We must first explain why these
organisms produce only a limited number of reproductive cells which are
all extruded at once, so that the _soma_ is rendered helpless. Why
should not the reproductive cells ripen in succession as they do
indirectly among the Monoplastides, that is to say in a succession of
generations, and as they do directly in great numbers among the
Metazoa? There would then be no necessity for the _soma_ to die, for a
few reproductive cells would always be present, and render the
persistence of the individual possible. In fact, the whole
arrangement—the formation of reproductive cells at one time only, and
their sudden extrusion,—presupposes the mortality of the somatic cells,
and is an adaptation to it, just as this mortality itself must be
regarded as an adaptation to the simultaneous ripening and sudden
extrusion of the generative cells. In short, there is no alternative to
the supposition stated above, viz. that the mortality of the somatic
cells arose with the differentiation of the originally homogeneous
cells of the Polyplastids into the dissimilar cells of the
Heteroplastids. And this is the first beginning of natural death.

Probably at first the somatic cells were not more numerous than the
reproductive cells, and while this was the case the phenomenon of death
was inconspicuous, for that which died was very small. But as the
somatic cells relatively increased, the body became of more importance
as compared with the reproductive cells, until death seems to affect
the whole individual, as in the higher animals, from which our ideas
upon the subject are derived. In reality, however, only one part
succumbs to natural death, but it is a part which in size far surpasses
that which remains and is immortal,—the reproductive cells.

Götte combats the statement that the idea of death necessarily implies
the existence of a corpse. Hence he maintains that the cellular sac
which is left after the extrusion of the reproductive cells among the
Orthonectides, and which ultimately dies, is not a corpse; ‘for it does
not represent the whole organism, any more than the isolated ectoderm
of any other Heteroplastid’ (l. c., p. 48). But it is only a popular
notion that a corpse must represent the entire organism. In cases of
violent death this idea is correct, because then the reproductive cells
are also killed. But as soon as we recognise that the reproductive
cells on the one side, and the somatic cells on the other, form
respectively the immortal and mortal parts of the Metazoan organism,
then we must acknowledge that only the latter,—that is, the _soma_
without the reproductive cells,—suffers natural death. The fact that
all the reproductive cells have not left the body (as sometimes
happens) before natural death takes place, does not affect this
conception. Among insects, for instance, it may happen that natural
death occurs before all the reproductive cells have matured, and these
latter then die with the _soma_. But this does not make any difference
to their potential immortality, any more than it modifies the
scientific conception of a corpse. The idea of natural death involves
that of a corpse, which consists of the _soma_, and when the latter
happens to contain reproductive cells, these do not succumb to a
natural death, which can never apply to them, but to an accidental
death. They are killed by the death of the _soma_ just as they might be
killed by any other accidental cause of death.

The scientific conception of a corpse is not affected, whether the dead
_soma_ remains whole for some time, or falls to pieces at once. I
cannot therefore agree with Götte when he denies that an Orthonectid
possesses ‘the possibility of becoming a corpse’ (in his sense of the
word) because ‘its death consists in the dissolution of the structure
of the organism.’ When the young of the Rhabdites form of _Ascaris
nigrovenosa_ bore through the body-walls of their parent, cause it to
disintegrate and finally devour it, the whole organism disappears, and
it would be difficult to say whether a corpse exists in the popular
sense of the word. But, scientifically speaking, there is certainly a
corpse; the real _soma_ of the animal dies, and this, however
subdivided, must be considered as a corpse. The fact that natural death
is so difficult to define without any accurate conception of what is
meant by a corpse, proves the necessity for arriving at a scientific
idea as to the meaning of the latter. There is no death without a
corpse—whether the latter be small or large, whole or in pieces.

If we compare the bodies of the higher Metazoa with those of the lower,
we see at once that not only has the structure of the body increased in
size and complexity as far as the _soma_ is concerned, but we also see
that another factor has been introduced, which exercises a most
important influence in lengthening the duration of life. This is the
replacement of cells by multiplication. Somatic cells have acquired (at
any rate in most tissues) the power of multiplying, after the body is
completely developed from the reproductive cells. The cells which have
undergone histological differentiation can increase by fission, and
thus supply the place of those which are being continually destroyed in
the course of metabolism. The difference between the higher and lower
Metazoa in this respect lies in the fact that there is only one
generation of somatic cells in the latter, and these are used up in the
process of metabolism at almost the same time that the reproductive
cells are extruded, while among the former there are successive
generations of somatic cells. I have elsewhere endeavoured to render
the duration of life in the animal kingdom intelligible by the
application of this principle, and have attempted to show that its
varying duration is determined in different species by the varying
number of somatic cell-generations[86]. Of course, the varying duration
of each cell-generation materially influences the total length of life,
and experience teaches us that the duration of cell-generations varies,
not only in the lowest Metazoa as compared with the highest, but even
in the various kinds of cells in one and the same species of animal.

We must, for the present, leave unanswered the question—upon what
changes in the physical constitution of protoplasm does the variation
in the capacity for cell-duration depend; and what are the causes which
determine the greater or smaller number of cell-generations. I mention
this obvious difficulty because it is the custom to meet every attempt
to search deeper into the common phenomena of life with the reproach
that so much is still left unexplained. If we must wait for the
explanation of these processes until we have ascertained the molecular
structure of cells, together with the changes that occur in this
structure and the consequences of the changes, we shall probably never
understand either the one or the other. The complex processes of life
can only be followed by degrees, and we can only hope to solve the
great problem by attacking it from all sides.

Therefore it is, in my opinion, an advance if we may assume that length
of life is dependent upon the number of generations of somatic cells
which can succeed one another in the course of a single life; and,
furthermore, that this number, as well as the duration of each single
cell-generation, is predestined in the germ itself. This view seems to
me to derive support from the obvious fact that the duration of each
cell-generation, and also the number of generations, undergo
considerable increase as we pass from the lowest to the highest Metazoa.

In an earlier work[87] I have attempted to show how exactly the
duration of life is adapted to the conditions by which it is
surrounded; how it is lengthened or shortened during the formation of
species, according to the conditions of life in each of them; in short,
how it is throughout an adaptation to these conditions. A few points
however were not touched upon in the work referred to, and these
require discussion; their consideration will also throw some light upon
the origin of natural death and the forms of life affected by it.

I have above explained the limited duration of the life of somatic
cells in the lower Metazoa—Orthonectides—as a phenomenon of adaptation,
and have ascribed it to the operation of natural selection, at the same
time pointing out that the existence of immortal Metazoan organisms is
conceivable. If the Monoplastides are able to multiply by fission,
through all time, then their descendants, in which division of labour
has induced the antithesis of reproductive and somatic cells, might
have done the same. If the Homoplastid cells reproduced their kind
uninterruptedly, equal powers of duration must have been possible for
the two kinds of Heteroplastid cells; they too might have been immortal
so far as immortality only depends upon the capacity for unlimited
reproduction.

But the capacity for existence possessed by any species is not only
dependent upon the power within it; it is also influenced by the
conditions of the external world, and this renders necessary the
process which we call adaptation. Thus it is just as inconceivable that
either a homogeneous or a heterogeneous cell-colony possessing the
physiological value of a multicellular individual should continue to
grow to an unlimited extent by continued cell-division, as it is
inconceivable that a unicellular being should increase in size to an
unlimited extent. In the latter case the process of cell-division
imposes a limit upon the size attained by growth. In the former, the
requirements of nutrition, respiration, and movement must prescribe a
limit to the growth of the cell-colony which constitutes the individual
of the higher species, just as in the case of the unicellular
Monoplastides, and it does not affect the argument if we consider this
limitation to be governed by the process of natural selection. It would
only be possible to regulate the relations of the single cells of the
colony to each other by fixing the number of cells within narrow
limits. During the development of _Magosphaera_—one of the
Homoplastides—the cells arrange themselves in the form of a hollow
sphere, lying in a gelatinous envelope. But the fact that reproduction
does not follow the simple unvarying rhythm of unicellular organisms is
of more importance; for a rhythm of a higher order appears, in which
each cell of the colony separates from its neighbours, when it has
reached a certain size, and proceeds by very rapid successive divisions
to give rise to a certain number of parts which arrange themselves as a
new colony. The number of divisions is controlled by the number of
cells to which the colony is limited, and at first this number may have
been very small. With the introduction of this secondary higher rhythm
during reproduction, the first germ of the Polyplastides became
evident; for then each process of fission was not, as in unicellular
organisms, equivalent to all the others; for in a colony of ten cells
the first fission differs from the second, third, or tenth, both in the
size of the products of division and also in remoteness from the end of
the process. This secondary fission is what we know as segmentation.

It seems to me of little importance whether the first process of
segmentation takes place in the water or within a cyst, although it is
quite possible that the necessity for some protective structure
appeared at a very early period, in order to shield the segmenting cell
from danger.

It is impossible to accept Götte’s conception of the germ (Keim), and
at this point the question arises as to its true meaning. I should
propose to include under this term every cell, cytode, or group of
cells which, while not possessing the structure of the mature
individual of the species, possesses the power of developing into it
under certain circumstances. The emphasis is now laid upon the
expression development, which is something opposed to simple growth,
without change of form. A cell which becomes a complete individual by
growth alone is not a germ but an individual, although a very small
one. For example, the small encapsuled Heliozoon, which arises as the
product of multiple fission, is not a germ in our sense of the word. It
is an individual, provided with all the characteristic marks of its
species, and it has only to protrude the retracted processes
(pseudopodia) and to take in the expelled water (formation of vacuoles)
in order to become capable of living in a free state. In this sense of
the word, germs are not confined to the Polyplastides, but are found in
many Monoplastides. There is nevertheless, in my opinion, a profound
and significant difference between the germs of these two groups. And
this lies not so much in the morphological as in the developmental
significance of these structures. As far as I have been able to compare
the facts, I may state that the germs of the Monoplastides are entirely
of secondary origin, and have never formed the phyletic origin of the
species in which they are found. For instance, the spore-formation of
the Gregarines resulted from a gradually increasing process of
division, which was concentrated into the period of encystment; and it
was induced by a necessity for rapid multiplication due to the
parasitic life and unfavourable surroundings of these animals. If
Gregarines were free-living animals, they would not need this method of
reproduction. The encysted animal would probably divide into eight,
four, or two parts, or perhaps, like many Infusoria[88], it would not
divide at all, so that the whole reproduction would depend on simple
fission alone during the free state.

The original mode of reproduction among the Monoplastides was
undoubtedly simple fission. This became connected with encystment,
which originally took place without multiplication; and only when the
divisions in the cyst became excessively numerous did such minute
plastids appear that a genuine process of development had to be
undergone in order to produce complete individuals. Here we have the
general conception of the germ as I defined it. Its limitations are
naturally not very sharply defined, for it is impossible to draw an
absolute distinction between simple growth and true development
accompanied by changes in form and structure. For instance, Häckel’s
_Protomyxa aurantiaca_ divides within its cyst into numerous plastids,
which might be spoken of as germs. But the changes of form which they
undergo before they become young _Protomyxae_ are very small, and for
the most part depend upon the expansion of the body, which existed in
the capsule as a contracted pear-shaped mass. It is therefore more
correct to speak only of the simple growth of the products of the
fission of the parent organism, and to look upon these products as
young _Protomyxae_ rather than germs. On the other hand, the young
animals which creep out of the germs (the ‘spores’) of _Gregarina
gigantea_, described by E. van Beneden, differ essentially from the
adult, and pass through a series of developmental stages before they
assume the characteristic form of a Gregarine.

This is true development[89]. But such a method of germ-formation and
development are found most frequently, although not exclusively, among
the parasitic Monoplastides, and this fact alone serves to indicate
their secondary origin. It is a form of ontogenetic development
differing from that of the Polyplastides in that it does not revert to
a phyletically primitive condition of the species, but, on the
contrary, exhibits stages which first appear in the phyletic
development of the specific form. The Psorosperms were only formed
after the Gregarines had become established as a group. The amoeboid
organisms which creep out of them are in no way to be regarded as the
primitive forms of the Gregarines, even if the latter may have
resembled them, but they are coenogenetic forms produced by the
necessity for a production of numerous and very minute germs. The
necessity for a process of genuine development perhaps depends upon the
small amount of material contained in one of these germs, and on other
conditions, such as change of host, change of medium, etc. It therefore
results that the fundamental law of biogenesis does not apply to the
Monoplastides; for these forms are either entirely without a genuine
ontogeny and only possess the possibility of growth, or else they are
only endowed with a coenogenetic ontogeny[90].

Some authorities may be inclined to limit the above proposition, and to
maintain that we must admit the possibility that we are likely to
occasionally meet with an ontogeny of which the stages largely
correspond with the most important stages in the phyletic development
of the species, and that the ontogenetic repetition of the phylogeny,
although not the rule, may still occur as a rare exception in the
Protozoa.

A careful consideration of the subject indicates, however, that the
occurrence of such an exception is very improbable. Such an ontogeny
would, for instance, occur if one of the lowest Monoplastides, such as
a Moneron, were to develope into a higher form, such as one of the
Flagellata, with mouth, eye-spot, and cortical layer, under such
external conditions that it would be advantageous for the existence of
its species that it should no longer reproduce itself by simple
fission, but that the periodical formation of a cyst (which was perhaps
previously part of the life-history) should be associated with the
occurrence of numerous divisions within the cyst itself, and with the
formation of germs. We must suppose either that these germs were so
minute that the young animals could not become Flagellata directly, or
that it was advantageous for them to move and feed as Monera at an
early period, and to assume the more complex structure of the parent by
gradual stages. In other words, the phyletic development would proceed
hand in hand with the ontogeny corresponding to it, although not from
any internal cause, but as an adaptation to the existing conditions of
life. But the supposed transformation of the species also depended upon
these same conditions of life, which must therefore have been of such a
nature as to bring about simultaneously, by an intercalation of germs
and by a genuine development, the evolution of the form in question in
the last stage of its ontogeny, and the maintenance of its original
condition during the initial stage. Such a combination of circumstances
can have scarcely ever happened. Against the occurrence of such a
transformation as we have supposed, it might be argued, indeed, that
the assumed production of very numerous germs does not occur among
free-living Monoplastides. Those which have acquired parasitic habits
must be younger phyletic forms, for their first host—whether a lowly or
a highly organized Metazoon—must have appeared before they could gain
access to it and adapt themselves to the conditions of a parasitic
life, and by this time the Flagellate Infusoria were already
established. It is by far less probable that the persistence or rather
the intercalation of the ancestral form would occur in an ontogenetic
cycle, consisting of a series of stages, and not of two only, as in our
example. For as soon as reproduction can be effected by the simple
fission of the adult, not only is there no reason why the earlier
phyletic stages should be again and again repeated, but such
recapitulation is simply impossible. We cannot, therefore, conclude
that the anomalous early stages of a Monoplastid such as _Acineta_
correspond with an early form of phyletic development.

Supposing, for instance, that the Acinetaria were derived from the
Ciliata, then this transformation must have taken place in the course
of the continued division of the ciliate ancestor—partially connected
with encystment, but for the most part independently of it. Of the
myriads of generations which such a process of development may have
occupied, perhaps the first set moved with suctorial processes, while
the second gradually adopted sedentary habits, and throughout the whole
of the long series, each succeeding generation must have been almost
exactly like its predecessor, and must always have consisted of
individuals which possessed the characters of the species.

This does not exclude the possibility that in spite of an assumed
sedentary mode of life, the need for locomotion and for obtaining food
in fresh places may have arisen at some period of life. But whenever
formation of swarm-spores takes place instead of simple fission, this
does not depend upon the persistence of an ancestral form in the
ontogenetic cycle, but is due to the intercalation of an entirely new
ontogenetic stage, which happens to resemble an ancestral form, in the
possession of cilia, etc.

I imagine that I have now sufficiently explained the above proposition,
that the repetition of the phylogeny in the ontogeny does not and
cannot occur among unicellular organisms.

With the Polyplastides the opposite is the case. There is no species,
as far as we know, which does not—either in each individual, or after
long cycles which comprise many individuals (alternation of
generations)—invariably revert to the Monoplastid state. This applies
from the lowest forms, such as _Magosphaera_ and the Orthonectides, up
to the very highest. In the latter a great number of intermediate
phyletic stages always occur, although some have been omitted as the
result of concentration in the ontogeny, while others have sometimes
been intercalated.

Sexual reproduction is the obvious cause of this very important
arrangement. Even if this is an hypothesis rather than a fact we must
nevertheless accept it unconditionally, because it is a method of
reproduction found everywhere. It is the rule in every group of the
animal kingdom, and is only absent in a few species in which it is
replaced by parthenogenesis. In these latter instances sexual
reproduction may be local, and entirely absent in certain districts
only (_Apus_), or it may be only apparently wanting; in some cases
where it is undoubtedly absent, it is equally certain that it was
present at an earlier period (_Limnadia Hermanni_). We cannot as yet
determine whether its loss will not involve the degeneration and
ultimate extinction of the species in question.

If the essential nature of sexual reproduction depends upon the
conjugation of two equivalent but dissimilar morphological elements,
then we can understand that a multicellular being can only attain
sexual reproduction when a unicellular stage is present in its
development; for the coalescence of entire multicellular organisms in
such a manner that fusion would only take place between equivalent
cells, would seem to be impracticable. In the necessity for sexual
reproduction, there is therefore also implied the necessity for
reverting to the original condition of the Polyplastides—that of a
single cell—and upon this alone depends the fundamental law of
biogenesis. This law is therefore confined to the Polyplastides, and
does not apply to the Monoplastides; and Götte’s suggestion that the
latter fall back into the primitive condition of the organism during
their encystment (rejuvenescence), finds no support in this aspect of
the question.

I have on a previous occasion[91] referred the utility of death to the
ultimate fact that the unending life of the Metazoan body would be a
useless luxury, and to the fact that the individuals would necessarily
become injured in the course of time, and would be therefore ‘not only
valueless to the species, but ... even harmful, for they take the place
of those which are sound’ (l. c., p. 24). I might also have said that
such damaged individuals would sooner or later fall victims to some
accidental death, so that there would be no possibility of real
immortality. I now propose to examine this statement a little more
closely, and to return to a question which has already been alluded to
before.

It is obvious that the advantages above set forth did not form the
motive which impelled natural selection to convert the immortal life of
the Monoplastides into the life of limited duration possessed by the
Heteroplastides, or more correctly, which led to the restriction of
potential immortality to the reproductive cells of the latter. It is at
any rate theoretically conceivable that a struggle might arise between
the mortal and immortal individuals of a certain Metazoan species, and
that natural selection might secure the success of the former, because
the longer the immortal individuals lived, the more defective they
became, and as a result gave rise to weaker offspring in diminished
numbers. Probably no one would be bold enough to suggest such a crude
example of natural selection. And yet I venture to think that the
principle of natural selection is here also to be taken into account,
and even plays, although in a negative rather than a positive way, a
very essential part in determining the duration of life in the Metazoa.

When the somatic cells of the first Heteroplastides ceased to be
immortal, such a loss would not in any way have precluded them from
regaining this condition. Just as, with the differentiation of the
first somatic cells of the lowest Heteroplastides, their duration was
limited to that of a single cell-generation,—so it must have been
possible for them, at a later period and if the necessity arose, to
lengthen their duration over two, three, or more generations. And if my
theory of the duration of life in the Metazoa is well founded, these
cells have as a matter of fact increased their duration, to an extent
about equal to that of the organism to which they belong. There is no
ground whatever for the assumption that it is impossible to fix the
number of cell-generations at infinity,—as actually happens in the case
of the reproductive cells,—but on the other hand it has already been
shown to be obvious that such an extension is opposed to the principle
of utility. It could never be to the advantage of a species to produce
crippled individuals, and therefore the infinite duration of
individuals has never reappeared among the Metazoa. So far the limited
duration of Metazoan life may be attributed to the worthlessness or
even the injurious nature of individuals, which although immortal, were
nevertheless liable to wear and tear. This fact explains why
immortality has never reappeared, it explains the predominance of
death, but it was not the single primary cause of this phenomenon. The
perishable and vulnerable nature of the _soma_ was the reason why
nature made no effort to endow this part of the individual with a life
of unlimited length.

Götte considers that death is inherent in reproduction, and in a
certain sense this is true, but not in the general way supposed by him.

I have endeavoured to show above that it is most advantageous for the
preservation of the species among the lowest Metazoa, that the body
should consist of a relatively small number of cells, and that the
reproductive cells should ripen simultaneously and all escape together.
If this conclusion be accepted, the uselessness of a prolonged life to
the somatic cells is obvious, and the occurrence of death at the time
of the extrusion of the reproductive cells is explained. In this manner
death (of the _soma_) and reproduction are here made to coincide.

This relation of reproduction to death still exists in a great number
of the higher animals. But such an association, together with the
simultaneous ripening of the reproductive cells, has not been
maintained continuously in the past. As the _soma_ becomes larger and
more highly organized, it is able to withstand more injuries, and its
average duration of life will extend: _pari passu_ with these changes
it will become increasingly advantageous not only for the number of
reproductive cells to be multiplied, but also for the time during which
they are produced to be prolonged. In this manner a lengthening of the
reproductive period arises, at first continuously and then
periodically. It is beyond my present purpose to consider in detail the
conditions upon which this lengthening depends, but I would emphasize
the fact that a lengthening of life is connected with the increase in
the duration of reproduction, while on the other hand there is no
reason to expect life to be prolonged beyond the reproductive period;
so that the end of this period is usually more or less coincident with
death.

A further prolongation of life could only take place when the parent
begins to undertake the duty of rearing the young. The most primitive
form of this is found among those animals, which do not expel their
reproductive cells as soon as they are ripe but retain them within
their bodies, so that the early stages of development take place under
the shelter of the parent organism. Associated with such a process
there is frequently a necessity for the germs to reach a certain spot,
where alone their further development can take place. Thus a segment of
a tapeworm lives until it has brought the embryos into a position which
affords the possibility of their passive transference to the stomach of
their special host. But the duration of life is first materially
lengthened when the offspring begin to be really tended, and as a
general rule the increase in length is exactly proportional to the time
which is demanded by the care of the young. Accurately conducted
observations are wanting upon this precise point, but the general
tendency of the facts, as a whole, cannot be doubted. Those insects of
which the care for their offspring terminates with the deposition of
eggs at the appropriate time, place, etc., do not survive this act; and
the duration of life in such imagos is shorter or longer according as
the eggs are laid simultaneously or ripen gradually. On the other hand,
insects—such as bees and ants—which tend their young, have a life which
is prolonged for years.

But the lengthening of the reproductive period alone may result in a
marked increase in the length of life, as is proved by the queen-bee.
In all these cases it is easy to imagine the operation of natural
selection in producing such alterations in the duration of life, and
indeed we might accurately calculate the amount of increase which would
be produced in any given case if the necessary data were available,
viz. the physiological strength of the body, and its relations to the
external world, such as, for instance, the power of obtaining food at
various periods of life, the expenditure of energy necessary for this
end, and the statistics of destruction, that is, the probabilities in
favour of the accidental death of a single individual at any given
time. These statistics must be known both for the imagos, larvae, and
eggs; for the lower they are for the imagos, and the higher for the
larvae and eggs, the more advantageous will it be, _ceteris paribus_,
for the number of eggs produced by the imago to be increased, and the
more probable it would therefore be that a long reproductive period,
involving a lengthening of the life of the imago, would be introduced.
But we are still far from being able to apply mathematics to the
phenomena of life; the factors are too numerous, and no attempt has
been made as yet to determine them with accuracy.

But we must at least admit the principle that both the lengthening and
shortening of life are possible by means of natural selection, and that
this process is alone able to render intelligible the exact adaptation
of the length of life to the conditions of existence.

A shortening of the normal duration of life is also possible; this is
shown in every case of sudden death, after the deposition of the whole
of the eggs at a single time. This occurs among certain insects, while
nearly allied forms of which the oviposition lasts over many days
therefore possess a correspondingly long imago-life. The _Ephemeridae_
and Lepidoptera afford many examples of this, and in an earlier work I
have collected some of them[92]. The humming-bird hawk-moth flies about
for weeks laying an egg here and there, and, like the allied poplar
hawk-moth and lime hawk-moth, probably dies when it has deposited all
the eggs which can be matured with the amount of nutriment at its
disposal. Many other Lepidoptera, such as the majority of butterflies,
fly about for weeks depositing their eggs, but others, such as the
emperor-moths and lappet-moths, lay their eggs one after another and
then die. The eggs of the parthenogenetic _Psychidae_ are laid directly
after the imago has left the cocoon, and death ensues immediately, so
that the whole life of the imago only lasts for a few hours. No one
could look upon this brief life as a primitive arrangement among
Lepidoptera, any more than we do upon the absence of wings in the
female _Psychidae_; shortening of life here is therefore clearly
explicable.

In such cases have we any right to speak of the fatal effect of
reproduction? We may certainly say that these insects die of
exhaustion; their vital strength is used up in the last effort of
laying eggs, and in the case of the males, in the act of copulation.
Reproduction is here certainly the most apparent cause of death, but a
more remote and deeper cause is to be found in the limitation of vital
strength to the length and the necessary duties of the reproductive
period. The fact that there are female Lepidoptera which, like the
emperor-moths, do not feed in the imago-state, proves the truth of this
statement. They still possess a mouth and a complete alimentary canal,
but they have no spiral ‘tongue,’ and do not take food of any kind, not
even a drop of water. They live in a torpid condition for days or weeks
until fertilization is accomplished, and then they lay their eggs and
die. The habit of extracting honey from flowers—common to most
hawk-moths and butterflies—would not have thus fallen into disuse, if
the store of nutriment, accumulated in the form of the fat-bodies,
during the life of the caterpillar, had not been exactly sufficient to
maintain life until the completion of oviposition. The fact that the
habit of taking food has been thus abandoned is a proof that the
duration of life beyond the reproductive period would not be to the
advantage of the species.

The protraction of existence into old age among the higher Metazoa
proves that death is not a necessary consequence of reproduction. It
seems to me that Götte’s statement ‘that the appearances of senility
must not be regarded as the general cause of death’ is not in
opposition to my opinions but rather to those which receive general
acceptance. I have myself pointed out that ‘death is not always
preceded by senility or a period of old age[93].’

The materials are wanting for a comprehensive investigation of the
causes which first introduced this period among the higher Metazoa; in
fact the most fundamental data are absent, for we do not even know the
part of the animal kingdom in which it first appeared: we cannot even
state the amount by which the duration of life exceeds that of the
period of reproduction, or what is the value to the species of this
last stage in the life of the individual.

It is in these general directions that we must seek for the
significance of old age. It is obviously of use to man, for it enables
the old to care for their children, and is also advantageous in
enabling the older individuals to participate in human affairs and to
exercise an influence upon the advancement of intellectual powers, and
thus to influence indirectly the maintenance of the race. But as soon
as we descend a step lower, if only as far as the apes, accurate facts
are wanting, for we are, and shall probably long be, ignorant of the
total duration of their life, and the point at which the period of
reproduction ceases.

                  *       *       *       *       *

I must here break off in the midst of these considerations, rather than
conclude them, for much still remains to be said. I hope, nevertheless,
that I have thrown new light upon some important points, and I now
propose to conclude with the following short abstract of the results of
my enquiry.

I. Natural death occurs only among multicellular beings; it is not
found among unicellular organisms. The process of encystment in the
latter is in no way comparable with death.

II. Natural death first appears among the lowest Heteroplastid Metazoa,
in the limitation of all the cells collectively to one generation, and
of the somatic or body-cells proper to a restricted period: the somatic
cells afterwards in the higher Metazoa came to last several and even
many generations, and life was lengthened to a corresponding degree.

III. This limitation went hand in hand with a differentiation of the
cells of the organism into reproductive and somatic cells, in
accordance with the principle of division of labour. This
differentiation took place by the operation of natural selection.

IV. The fundamental biogenetic law applies only to multicellular
beings; it does not apply to unicellular forms of life. This depends on
the one hand upon the mode of reproduction by fission which obtains
among the Monoplastides (unicellular organisms), and on the other upon
the necessity, induced by sexual reproduction, for the maintenance of a
unicellular stage in the development of the Polyplastides
(multicellular organisms).

V. Death itself, and the longer or shorter duration of life, both
depend entirely on adaptation. Death is not an essential attribute of
living matter; it is neither necessarily associated with reproduction,
nor a necessary consequence of it.

                  *       *       *       *       *

In conclusion, I should wish to call attention to an idea which is
rather implied than expressed in this essay:—it is, that reproduction
did not first make its appearance coincidently with death. Reproduction
is in truth an essential attribute of living matter, just as is the
growth which gives rise to it. It is as impossible to imagine life
enduring without reproduction as it would be to conceive life lasting
without the capacity for absorption of food and without the power of
metabolism. Life is continuous and not periodically interrupted: ever
since its first appearance upon the earth, in the lowest organisms, it
has continued without break; the forms in which it is manifested have
alone undergone change. Every individual alive to-day—even the very
highest—is to be derived in an unbroken line from the first and lowest
forms.

------------------------------------------------------------------------

                       Footnotes for Chapter III.

Footnote 59:

  ‘Ueber den Ursprung des Todes,’ Hamburg and Leipzig, 1883.

Footnote 60:

  As in the case of the bodies of monks on the Great St. Bernard, or
  the dried-up bodies in the well-known Capuchine Monastery at Palermo.

Footnote 61:

  Professor Gruber informs me that among the Infusoria of the harbour
  of Genoa, he has observed a species which encysts upon one of the
  free-swimming Copepoda. He has often found as many as ten cysts upon
  one of these Copepods, and has observed the escape of their contents
  whenever the water under the cover-glass began to putrefy. Here
  advantage is probably gained in the rapid transport of the cyst by
  the Crustacean.

Footnote 62:

  The views of most biologists who have worked at this subject agree in
  all essentials with that expressed above. Bütschli says (Bronn’s
  ‘Klassen und Ordnungen des Thierreichs,’ Protozoa, p. 148): ‘The
  process of encystment does not appear to have originally borne any
  direct relation to reproduction: it appears on the contrary to have
  taken place originally,—as it frequently does at the present
  day,—either for the protection of the organism against injurious
  external influences, such as desiccation or the fatal effects of
  impure water, etc.; and also to enable the organism, after taking up
  an unusually abundant supply of food, to assimilate it in safety.’
  Balbiani (‘Journ. de Micrographie,’ Tom. V. 1881, p. 293) says in
  reference to the Infusoria, ‘Un petit nombre d’espèces, au lieu de se
  multiplier à l’état de vie active, se reproduisent dans une sorte
  d’état de repos, dit état d’enkystement. Ces sortes de kystes peuvent
  être désignés sous le nom de kystes de reproduction, par opposition
  avec d’autres kystes, dans lesquels les Infusoires se renferment pour
  se soustraire à des conditions devenues défavorables du milieu qu’ils
  habitent, le manque d’air, le dessèchement, etc.—ceux-ci sont des
  kystes de conservation....’

Footnote 63:

  This is of importance in so far as single individuals might be thus
  compelled to encyst even when the existing external conditions of
  life do not require it. The substance which _Actinosphaerium_, for
  example, employs in the secretion of its thick siliceous cyst must
  have been gradually accumulated by means of a process peculiar to the
  species. We can scarcely be in error if we assume that the silica
  accumulated in the organism cannot increase to an unlimited extent
  without injury to the other vital processes and that the secretion of
  the cyst must take place as soon as the accumulation has exceeded a
  certain limit. Thus we can understand that encystment may occur
  without any external necessity. Similarly, certain Entomostraca (_e.
  g._ _Moina_) produce winter-eggs in a particular generation, and
  these are formed even when the animals are kept in a room protected
  from cold and desiccation.

Footnote 64:

  Upon this point Professor Gruber intends to publish an elaborate
  memoir.

Footnote 65:

  This view has not even been proved for _Actinosphaerium_, upon which
  Götte chiefly relies. The observations which we now possess merely
  indicate that the animal contracts to the smallest volume possible.
  Compare F. E. Schulze, ‘Rhizopodenstudien,’ I, Arch. f. mikr. Anat.
  Bd. 10, p. 328; and Karl Brandt, ‘Ueber Actinosphaerium Eichhornii,’
  Inaug. Diss.; Halle, 1877.

Footnote 66:

  The conception of Protozoa and Metazoa does not correspond exactly
  with that of unicellular and multicellular beings, for which Götte
  has proposed the names Mono- and Polyplastides.

Footnote 67:

  Among the Rhizopoda encystment is only known in fresh-water forms,
  and not in a single one of the far more numerous marine forms which
  possess shells (see Bütschli, ‘Protozoa,’ p. 148); the marine
  Rhizopoda are not exposed to the effects of desiccation or frost, and
  thus the strongest motives for the process of encystment do not
  exist, at least among forms possessing a shell.

Footnote 68:

  I trust that it will not be objected that the germ-cells cannot be
  immortal, because they frequently perish in large numbers, as a
  result of the natural death of the individual. There are certain
  definite conditions under which alone a germ-cell can render its
  potential immortality actual, and these conditions are for the most
  part fulfilled with difficulty (fertilization, etc.). It follows from
  this fact that the germ-cells must always be produced in numbers
  which reach some very high multiple of the necessary number of
  offspring, if these latter are to be ensured for the species. If in
  the natural death of the individual the germ-cells must also die, the
  _natural_ death of the _soma_ becomes a cause of _accidental_ death
  to the germ-cells.

Footnote 69:

  l. c., p. 78.

Footnote 70:

  l. c., p. 47.

Footnote 71:

  ‘Entwicklungsgeschichte der Unke,’ Leipzig, 1875, p. 65.

Footnote 72:

  Id., p. 842.

Footnote 73:

  ‘Ursprung des Todes,’ p. 79.

Footnote 74:

  l. c., p. 42.

Footnote 75:

  ‘Contributions à l’histoire des Mesozoaires. Recherches sur
  l’organisation et le développement embryonnaire des Orthonectides,’
  Arch. de Biologie, vol. iii. 1882.

Footnote 76:

  l. c., p. 37.

Footnote 77:

  Julin does not enter into further details on this point, and it is
  not quite clear at what precise time the cells of the ectoderm
  atrophy; but this is irrelevant to the origin of death, since the
  granular mass surrounding the egg-cells at any rate belongs to the
  _soma_ of the mother.

Footnote 78:

  Leuckart finds such a great resemblance between the newly born young
  of _Distoma_ and the Orthonectides, that he is inclined to believe
  that the latter are Trematodes, ‘which in spite of sexual maturity
  have not developed further than the embryonic condition of the
  _Distoma_’  (‘Zur Entwicklungsgeschichte des Leberegels,’ Zool.
  Anzeiger, 1881, No. 99). In reference to the Dicyemidae, which
  resemble the Orthonectides in their manner of living and in their
  structure, Gegenbaur has stated his opinion that they belong to a
  ‘stage in the development of Platyhelminthes’ (Grundriss d.
  vergleich. Anatomie). Giard includes both in the ‘phylum Vermes,’ and
  regards them as much degenerated by parasitism; and Whitman—the
  latest investigator of the Dicyemids—speaks of them in a similar
  manner in his excellent work ‘Contributions to the Life-history and
  Classification of Dicyemids’ (Leipzig, 1882).

Footnote 79:

  ‘Dauer des Lebens;’ translated as the first essay in this volume.

Footnote 80:

  See the first essay upon ‘The Duration of Life,’ p. 22 et seq.

Footnote 81:

  ‘Ursprung des Todes,’ p. 29.

Footnote 82:

  l. c., p. 5.

Footnote 83:

  See the preceding essay ‘On Heredity.’

Footnote 84:

  The problem is very easily solved if we seek assistance from the
  principle of panmixia developed in the second essay ‘On Heredity.’ As
  soon as natural selection ceases to operate upon any character,
  structural or functional, it begins to disappear. As soon, therefore,
  as the immortality of somatic cells became useless they would begin
  to lose this attribute. The process would take place more quickly, as
  the histological differentiation of the somatic cells became more
  useful and complete, and thus became less compatible with their
  everlasting duration.—A. W. 1888.

Footnote 85:

  See the preceding essay ‘On Heredity.’

Footnote 86:

  See the first essay on ‘The Duration of Life.’

Footnote 87:

  See the first essay on ‘The Duration of Life.’

Footnote 88:

  These assumptions can be authenticated among the Infusoria. The
  encysted _Colpoda cucullus_, Ehrbg. divides into two, four, eight, or
  sixteen parts; _Otostoma Carteri_, into two, four, or eight; _Tillina
  magna_, Gruber, into four or five; _Lagynus_ sp. Gruber, into two;
  _Amphileptus meleagris_, Ehrbg. into two or four. The last two
  species and many others frequently do not divide at all during the
  encysted condition. But while any further increase in the number of
  divisions within the cyst does not occur in free-swimming Infusoria,
  the interesting case of _Ichthyophthirius multifiliis_, Fouquet,
  shows that parasitic habits call forth a remarkable increase in the
  number of divisions. This animal divides into at least a thousand
  daughter individuals.

Footnote 89:

  True development also takes place in the above-mentioned
  _Ichthyophthirius_. While in other Infusoria the products of fission
  exactly resemble the parent, in _Ichthyophthirius_ they have a
  different form; the sucking mouth is wanting while provisional
  clasping cilia are at first present. In this case therefore the word
  germ may be rightly applied, and _Ichthyophthirius_ affords an
  interesting example of the phyletic origin of germs among the lower
  Flagellata and Gregarines. Cf. Fouquet, ‘Arch. Zool. Expérimentale,’
  Tom. V. p. 159. 1876.

Footnote 90:

  Bütschli, long ago, doubted the application of the fundamental law of
  biogenesis to the Protozoa (cf. ‘Ueber die Entstehung der
  Schwärmsprösslings der Podophrya quadripartita,’ Jen. Zeit. f. Med.
  u. Naturw. Bd. X. p. 19, Note). Gruber has more recently expressed
  similar views, and in fact denies the presence of development in the
  Protozoa, and only recognizes growth (‘Dimorpha mutans, Z. f. W. Z.’
  Bd. XXXVII. p. 445). This proposition must however be restricted,
  inasmuch as a development certainly occurs, although one which is
  coenogenetic and not palingenetic.

Footnote 91:

  See the first essay on ‘The Duration of Life,’ p. 23 _et seq._

Footnote 92:

  See Appendix to the first essay on ‘The Duration of Life,’ pp. 43-46.

Footnote 93:

  See the first essay on ‘The Duration of Life,’ p. 21.

------------------------------------------------------------------------







                                  IV.

                    THE CONTINUITY OF THE GERM-PLASM
               AS THE FOUNDATION OF A THEORY OF HEREDITY.

                                 1885.




------------------------------------------------------------------------

                   CONTINUITY OF THE GERM-PLASM, &c.


                                PREFACE.

The ideas developed in this essay were first expressed during the past
winter in a lecture delivered to the students of this University
(Freiburg), and they were shortly afterwards—in February and the
beginning of March—written in their present form. I mention this,
because I might otherwise be reproached for a somewhat partial use of
the most recent publications on related subjects. Thus I did not
receive Oscar Hertwig’s paper—‘Contributions to the Theory of Heredity’
(Zur Theorie der Vererbung), until after I had finished writing my
essay, and I could not therefore make as much use of it as I should
otherwise have done. Furthermore, the paper by Kölliker on ‘The
Significance of the Nucleus in the Phenomena of Heredity’ (Die
Bedeutung der Zellkerne für die Vorgänge der Vererbung), did not appear
until after the completion of my manuscript. The essential treatment of
the subject would not, however, have been altered if I had received the
papers at an earlier date, for as far as the most important point—the
significance of the nucleus—is concerned, my views are in accordance
with those of both the above-named investigators; while the points upon
which our views do not coincide had already received attention in the
manuscript.

                                                                   A. W.

      Freiburg I. Breisgau,

                    June 16, 1885.






------------------------------------------------------------------------

                   CONTINUITY OF THE GERM-PLASM, &c.

                               CONTENTS.

  Introduction                                                     165

  I. The Germ-Plasm                                                174

     1. Historical development of the theory as to the             174
        localization of the germ-plasm in the nucleus

     2. Nägeli’s ‘idioplasm’ is not identical with Weismann’s      180
        ‘germ-plasm’

     3. A retransformation of somatic idioplasm into germ-         183
        idioplasm does not take place

     4. Confirmation of the theory as to the significance of the   185
        nuclear substance afforded by Nussbaum’s and Gruber’s
        experiments on regeneration in Infusoria

     5. The nucleoplasm changes during ontogeny according to a     186
        certain law

     6. The identity of the daughter-nuclei produced by indirect   187
        nuclear division, as assumed by Strasburger, is not
        necessary for my theory

     7. The gradual decrease in complexity of the structure of     190
        the nucleus during ontogeny

     8. Nägeli’s view on the germs (‘Anlagen’) in the idioplasm    192

     9. The manner in which germ-cells arise from somatic cells    194

    10. ’Embryonic’ cells in the mature organism                   196

    11. The rule of probability is against a retransformation of   198
        somatic idioplasm into germ-plasm

    12. The regular cyclical development of the idioplasm          199
        founded upon phylogeny by Nägeli

    13. It follows from phyletic considerations that the germ-     201
        cells have not arisen at the end of ontogeny

    14. They originally arose at the beginning of ontogeny, but    202
        at a later period the time of their origin was displaced

    15. A continuity of the germ-cells does not now exist in       205
        most cases

    16. But there is a continuity of the germ-plasm                205

    17. Strasburger’s objection to my supposition that the germ-   209
        plasm passes along distinct routes

    18. The cell-body may remain unchanged when the nucleus is     210
        changed

    19. It is conceivable that all somatic nuclei may contain      211
        some germ-plasm

  II. The Significance of the Polar Bodies                         212

     1. The egg-cell contains two kinds of idioplasm; germ-plasm   213
        and histogenetic nucleoplasm

     2. The expulsion of the polar bodies signifies the removal    214
        of the histogenetic nucleoplasm

     3. Other theories as to the significance of the polar         214
        bodies

     4. The modes of occurrence of polar bodies                    217

     5. Their possible occurrence in male germ-cells               219

     6. There are two kinds of nucleoplasm in the male germ-       219
        cells

     7. Polar bodies in plants                                     222

     8. Morphological origin of polar bodies                       223

  III. On the Nature of Parthenogenesis                            225

     1. The phenomena exhibited in the maturation of the egg are   225
        identical in parthenogenetic and sexual development

     2. The difference between parthenogenetic and sexual cells    226
        must be of a quantitative nature

     3. The quantity of the germ-plasm determines development      227

     4. The expulsion of polar bodies depends upon the             230
        antagonism between germ-plasm and ovogenetic nucleoplasm

     5. Fertilization does not act dynamically                     231

     6. An insufficient quantity of germ-plasm arrests             232
        development

     7. Relation of the nucleus to the cell                        234

     8. The case of the bee does not constitute any objection to   234
        my theory

     9. Strasburger’s views upon parthenogenesis                   237

    10. Parthenogenesis does not depend upon abundant nutrition    239

    11. The indirect causes of sexual and parthenogenetic          241
        reproduction

    12. The direct causes                                          242

    13. Explanation of the formation of nutritive cells            243

    14. Identity of the germ-plasm in male and female germ-cells   246

  Note                                                             249






------------------------------------------------------------------------




                                  IV.

                THE CONTINUITY OF THE GERM-PLASM AS THE
                  FOUNDATION OF A THEORY OF HEREDITY.


                             Introduction.

When we see that, in the higher organisms, the smallest structural
details, and the most minute peculiarities of bodily and mental
disposition, are transmitted from one generation to another; when we
find in all species of plants and animals a thousand characteristic
peculiarities of structure continued unchanged through long series of
generations; when we even see them in many cases unchanged throughout
whole geological periods; we very naturally ask for the causes of such
a striking phenomenon: and enquire how it is that such facts become
possible, how it is that the individual is able to transmit its
structural features to its offspring with such precision. And the
immediate answer to such a question must be given in the following
terms:—‘A single cell out of the millions of diversely differentiated
cells which compose the body, becomes specialized as a sexual cell; it
is thrown off from the organism and is capable of reproducing all the
peculiarities of the parent body, in the new individual which springs
from it by cell-division and the complex process of differentiation.’
Then the more precise question follows: ‘How is it that such a single
cell can reproduce the _tout ensemble_ of the parent with all the
faithfulness of a portrait?’

The answer is extremely difficult; and no one of the many attempts to
solve the problem can be looked upon as satisfactory; no one of them
can be regarded as even the beginning of a solution or as a secure
foundation from which a complete solution may be expected in the
future. Neither Häckel’s[94], ‘Perigenesis of the Plastidule,’ nor
Darwin’s[95] ‘Pangenesis,’ can be regarded as such a beginning. The
former hypothesis does not really treat of that part of the problem
which is here placed in the foreground, viz. the explanation of the
fact that the tendencies of heredity are present in single cells, but
it is rather concerned with the question as to the manner in which it
is possible to conceive the transmission of a certain tendency of
development into the sexual cell, and ultimately into the organism
arising from it. The same may be said of the hypothesis of His[96],
who, like Häckel, regards heredity as the transmission of certain kinds
of motion. On the other hand, it must be conceded that Darwin’s
hypothesis goes to the very root of the question, but he is content to
give, as it were, a provisional or purely formal solution, which, as he
himself says, does not claim to afford insight into the real phenomena,
but only to give us the opportunity of looking at all the facts of
heredity from a common standpoint. It has achieved this end, and I
believe it has unconsciously done more, in that the thoroughly logical
application of its principles has shown that the real causes of
heredity cannot lie in the formation of gemmules or in any allied
phenomena. The improbabilities to which any such theory would lead are
so great that we can affirm with certainty that its details cannot
accord with existing facts. Furthermore, Brooks’[97] well-considered
and brilliant attempt to modify the theory of Pangenesis, cannot escape
the reproach that it is based upon possibilities, which one might
certainly describe as improbabilities. But although I am of opinion
that the whole foundation of the theory of Pangenesis, however it may
be modified, must be abandoned, I think, nevertheless, its author
deserves great credit, and that its production has been one of those
indirect roads along which science has been compelled to travel in
order to arrive at the truth. Pangenesis is a modern revival of the
oldest theory of heredity, that of Democritus, according to which the
sperm is secreted from all parts of the body of both sexes during
copulation, and is animated by a bodily force; according to this theory
also, the sperm from each part of the body reproduces the same part[98].

If, according to the received physiological and morphological ideas of
the day, it is impossible to imagine that gemmules produced by each
cell of the organism are at all times to be found in all parts of the
body, and furthermore that these gemmules are collected in the sexual
cells, which are then able to again reproduce in a certain order each
separate cell of the organism, so that each sexual cell is capable of
developing into the likeness of the parent body; if all this is
inconceivable, we must enquire for some other way in which we can
arrive at a foundation for the true understanding of heredity. My
present task is not to deal with the whole question of heredity, but
only with the single although fundamental question—‘How is it that a
single cell of the body can contain within itself all the hereditary
tendencies of the whole organism?’ I am here leaving out of account the
further question as to the forces and the mechanism by which these
tendencies are developed in the building-up of the organism. On this
account I abstain from considering at present the views of Nägeli, for
as will be shown later on, they only slightly touch this fundamental
question, although they may certainly claim to be of the highest
importance with respect to the further question alluded to above.

Now if it is impossible for the germ-cell to be, as it were, an extract
of the whole body, and for all the cells of the organism to despatch
small particles to the germ-cells, from which the latter derive their
power of heredity; then there remain, as it seems to me, only two other
possible, physiologically conceivable, theories as to the origin of
germ-cells, manifesting such powers as we know they possess. Either the
substance of the parent germ-cell is capable of undergoing a series of
changes which, after the building-up of a new individual, leads back
again to identical germ-cells; or the germ-cells are not derived at
all, as far as their essential and characteristic substance is
concerned, from the body of the individual, but they are derived
directly from the parent germ-cell.

I believe that the latter view is the true one: I have expounded it for
a number of years, and have attempted to defend it, and to work out its
further details in various publications. I propose to call it the
theory of ‘The Continuity of the Germ-plasm,’ for it is founded upon
the idea that heredity is brought about by the transference from one
generation to another, of a substance with a definite chemical, and
above all, molecular constitution. I have called this substance
‘germ-plasm,’ and have assumed that it possesses a highly complex
structure, conferring upon it the power of developing into a complex
organism. I have attempted to explain heredity by supposing that in
each ontogeny, a part of the specific germ-plasm contained in the
parent egg-cell is not used up in the construction of the body of the
offspring, but is reserved unchanged for the formation of the
germ-cells of the following generation.

It is clear that this view of the origin of germ-cells explains the
phenomena of heredity very simply, inasmuch as heredity becomes thus a
question of growth and of assimilation,—the most fundamental of all
vital phenomena. If the germ-cells of successive generations are
directly continuous, and thus only form, as it were, different parts of
the same substance, it follows that these cells must, or at any rate
may, possess the same molecular constitution, and that they would
therefore pass through exactly the same stages under certain conditions
of development, and would form the same final product. The hypothesis
of the continuity of the germ-plasm gives an identical starting-point
to each successive generation, and thus explains how it is that an
identical product arises from all of them. In other words, the
hypothesis explains heredity as part of the underlying problems of
assimilation and of the causes which act directly during ontogeny: it
therefore builds a foundation from which the explanation of these
phenomena can be attempted.

It is true that this theory also meets with difficulties, for it seems
to be unable to do justice to a certain class of phenomena, viz. the
transmission of so-called acquired characters. I therefore gave
immediate and special attention to this point in my first publication
on heredity[99], and I believe that I have shown that the hypothesis of
the transmission of acquired characters—up to that time generally
accepted—is, to say the least, very far from being proved, and that
entire classes of facts which have been interpreted under this
hypothesis may be quite as well interpreted otherwise, while in many
cases they must be explained differently. I have shown that there is no
ascertained fact, which, at least up to the present time, remains in
irrevocable conflict with the hypothesis of the continuity of the
germ-plasm; and I do not know any reason why I should modify this
opinion to-day, for I have not heard of any objection which appears to
be feasible. E. Roth[100] has objected that in pathology we everywhere
meet with the fact that acquired local disease may be transmitted to
the offspring as a predisposition; but all such cases are exposed to
the serious criticism that the very point that first needs to be placed
on a secure footing is incapable of proof, viz. the hypothesis that the
causes which in each particular case led to the predisposition, were
really acquired. It is not my intention, on the present occasion, to
enter fully into the question of acquired characters; I hope to be able
to consider the subject in greater detail at a future date. But in the
meantime I should wish to point out that we ought, above all, to be
clear as to what we really mean by the expression ‘acquired character.’
An organism cannot acquire anything unless it already possesses the
predisposition to acquire it: acquired characters are therefore no more
than local or sometimes general variations which arise under the
stimulus provided by certain external influences. If by the
long-continued handling of a rifle, the so-called ‘Exercierknochen’ (a
bony growth caused by the pressure of the weapon in drilling) is
developed, such a result depends upon the fact that the bone in
question, like every other bone, contains within itself a
predisposition to react upon certain mechanical stimuli, by growth in a
certain direction and to a certain extent. The predisposition towards
an ‘Exercierknochen’ is therefore already present, or else the growth
could not be formed; and the same reasoning applies to all other
‘acquired characters.’

Nothing can arise in an organism unless the predisposition to it is
pre-existent, for every acquired character is simply the reaction of
the organism upon a certain stimulus. Hence I should never have thought
of asserting that predispositions cannot be transmitted, as E. Roth
appears to believe. For instance, I freely admit that the
predisposition to an ‘Exercierknochen’ varies, and that a strongly
marked predisposition may be transmitted from father to son, in the
form of bony tissue with a more susceptible constitution. But I should
deny that the son could develope an ‘Exercierknochen’ without having
drilled, or that, after having drilled, he could develope it more
easily than his father, on account of the drilling through which the
latter first acquired it. I believe that this is as impossible as that
the leaf of an oak should produce a gall, without having been pierced
by a gall-producing insect, as a result of the thousands of antecedent
generations of oaks which have been pierced by such insects, and have
thus ‘acquired’ the power of producing galls. I am also far from
asserting that the germ-plasm—which, as I hold, is transmitted as the
basis of heredity from one generation to another—is absolutely
unchangeable or totally uninfluenced by forces residing in the organism
within which it is transformed into germ-cells. I am also compelled to
admit that it is conceivable that organisms may exert a modifying
influence upon their germ-cells, and even that such a process is to a
certain extent inevitable. The nutrition and growth of the individual
must exercise some influence upon its germ-cells; but in the first
place this influence must be extremely slight, and in the second place
it cannot act in the manner in which it is usually assumed that it
takes place. A change of growth at the periphery of an organism, as in
the case of an ‘Exercierknochen,’ can never cause such a change in the
molecular structure of the germ-plasm as would augment the
predisposition to an ‘Exercierknochen,’ so that the son would inherit
an increased susceptibility of the bony tissue or even of the
particular bone in question. But any change produced will result from
the reaction of the germ-cell upon changes of nutrition caused by
alteration in growth at the periphery, leading to some change in the
size, number, or arrangement of its molecular units. In the present
state of our knowledge there is reason for doubting whether such
reaction can occur at all; but, if it can take place, at all events the
quality of the change in the germ-plasm can have nothing to do with the
quality of the acquired character, but only with the way in which the
general nutrition is influenced by the latter. In the case of the
‘Exercierknochen’ there would be practically no change in the general
nutrition, but if such a bony growth could reach the size of a
carcinoma, it is conceivable that a disturbance of the general
nutrition of the body might ensue. Certain experiments on plants—in
which Nägeli showed that they can be submitted to strongly varied
conditions of nutrition for several generations, without the production
of any visible hereditary change—show that the influence of nutrition
upon the germ-cells must be very slight, and that it may possibly leave
the molecular structure of the germ-plasm altogether untouched. This
conclusion is also supported by comparing the uncertainty of these
results with the remarkable precision with which heredity acts in the
case of those characters which are known to be transmitted. In fact, up
to the present time, it has never been proved that any changes in
general nutrition can modify the molecular structure of the germ-plasm,
and far less has it been rendered by any means probable that the
germ-cells can be affected by acquired changes which have no influence
on general nutrition. If we consider that each so-called predisposition
(that is, a power of reacting upon a certain stimulus in a certain way,
possessed by any organism or by one of its parts) must be innate, and
further that each acquired character is only the predisposed reaction
of some part of an organism upon some external influence; then we must
admit that only one of the causes which produce any acquired character
can be transmitted, the one which was present before the character
itself appeared, viz. the predisposition; and we must further admit
that the latter arises from the germ, and that it is quite immaterial
to the following generation whether such predisposition comes into
operation or not. The continuity of the germ-plasm is amply sufficient
to account for such a phenomenon, and I do not believe that any
objection to my hypothesis, founded upon the actually observed
phenomena of heredity, will be found to hold. If it be accepted, many
facts will appear in a light different from that which has been cast
upon them by the hypothesis which has been hitherto received,—a
hypothesis which assumes that the organism produces germ-cells afresh,
again and again, and that it produces them entirely from its own
substance. Under the former theory the germ-cells are no longer looked
upon as the product of the parent’s body, at least as far as their
essential part—the specific germ-plasm—is concerned: they are rather
considered as something which is to be placed in contrast with the
_tout ensemble_ of the cells which make up the parent’s body, and the
germ-cells of succeeding generations stand in a similar relation to one
another as a series of generations of unicellular organisms, arising by
a continued process of cell-division. It is true that in most cases the
generations of germ-cells do not arise immediately from one another as
complete cells, but only as minute particles of germ-plasm. This latter
substance, however, forms the foundation of the germ-cells of the next
generation, and stamps them with their specific character. Previous to
the publication of my theory, G. Jäger[101], and later M.
Nussbaum[102], have expressed ideas upon heredity which come very near
to my own[103]. Both of these writers started with the hypothesis that
there must be a direct connexion between the germ-cells of succeeding
generations, and they tried to establish such a continuity by supposing
that the germ-cells of the offspring are separated from the parent
germ-cell before the beginning of embryonic development, or at least
before any histological differentiation has taken place. In this form
their suggestion cannot be maintained, for it is in conflict with
numerous facts. A continuity of the germ-_cells_ does not now take
place, except in very rare instances; but this fact does not prevent us
from adopting a theory of the continuity of the germ-_plasm_, in favour
of which much weighty evidence can be brought forward. In the following
pages I shall attempt to develope further the theory of which I have
just given a short account, to defend it against any objections which
have been brought forward, and to draw from it new conclusions which
may perhaps enable us more thoroughly to appreciate facts which are
known, but imperfectly understood. It seems to me that this theory of
the continuity of the germ-plasm deserves at least to be examined in
all its details, for it is the simplest theory upon the subject, and
the one which is most obviously suggested by the facts of the case, and
we shall not be justified in forsaking it for a more complex theory
until proof that it can be no longer maintained is forthcoming. It does
not presuppose anything except facts which can be observed at any
moment, although they may not be understood,—such as assimilation, or
the development of like organisms from like germs; while every other
theory of heredity is founded on hypotheses which cannot be proved. It
is nevertheless possible that continuity of the germ-plasm does not
exist in the manner in which I imagine that it takes place, for no one
can at present decide whether all the ascertained facts agree with and
can be explained by it. Moreover the ceaseless activity of research
brings to light new facts every day, and I am far from maintaining that
my theory may not be disproved by some of these. But even if it should
have to be abandoned at a later period, it seems to me that, at the
present time, it is a necessary stage in the advancement of our
knowledge, and one which must be brought forward and passed through,
whether it prove right or wrong, in the future. In this spirit I offer
the following considerations, and it is in this spirit that I should
wish them to be received.

                           I. The Germ-plasm.

I must first define precisely the exact meaning of the term germ-plasm.

In my previous writings in which the subject has been alluded to, I
have simply spoken of germ-plasm without indicating more precisely the
part of the cell in which we may expect to find this substance—the
bearer of the characteristic nature of the species and of the
individual. In the first place such a course was sufficient for my
immediate purpose, and in the second place the number of ascertained
facts appeared to be insufficient to justify a more exact definition. I
imagined that the germ-plasm was that part of a germ-cell of which the
chemical and physical properties—including the molecular
structure—enable the cell to become, under appropriate conditions, a
new individual of the same species. I therefore believed it to be some
such substance as Nägeli[104], shortly afterwards, called idioplasm,
and of which he attempted, in an admirable manner, to give us a clear
understanding. Even at that time one might have ventured to suggest
that the organized substance of the nucleus is in all probability the
bearer of the phenomena of heredity, but it was impossible to speak
upon this point with any degree of certainty. O. Hertwig[105] and
Fol[106] had shown that the process of fertilization is attended by a
conjugation of nuclei, and Hertwig had even then distinctly said that
fertilization generally depends upon the fusion of two nuclei; but the
possibility of the co-operation of the substance of the two germ-cells
could not be excluded, for in all the observed cases the sperm-cell was
very small and had the form of a spermatozoon, so that the amount of
its cell-body, if there is any, coalescing with the female cell, could
not be distinctly seen, nor was it possible to determine the manner in
which this coalescence took place. Furthermore, it was for some time
very doubtful whether the spermatozoon really contained true nuclear
substance, and even in 1879 Fol was forced to the conclusion that these
bodies consist of cell-substance alone. In the following year my
account of the sperm-cells of _Daphnidae_ followed, and this should
have removed every doubt as to the cellular nature of the sperm-cells
and as to their possession of an entirely normal nucleus, if only the
authorities upon the subject had paid more attention to these
statements[107]. In the same year (1880) Balfour summed up the facts in
the following manner—‘The act of impregnation may be described as the
fusion of the ovum and spermatozoon, and the most important feature in
this act appears to be the fusion of a male and female nucleus[108].’
It is true that Calberla had already observed in _Petromyzon_, that the
tail of the spermatozoon does not penetrate into the egg, but remains
in the micropyle; but on the other hand the head and part of the
‘middle-piece’ which effect fertilization, certainly contain a small
fraction of the cell-body in addition to the nuclear substance, and
although the amount of the former which thus enters the egg must be
very small, it might nevertheless be amply sufficient to transmit the
tendencies of heredity. Nägeli and Pflüger rightly asserted, at a later
date, that the amount of the substance which forms the basis of
heredity is necessarily very small, for the fact that hereditary
tendencies are as strong on the paternal as on the maternal side,
forces us to assume that the amount of this substance is nearly equal
in both male and female germ-cells. Although I had not published
anything upon the point, I was myself inclined to ascribe considerable
importance to the cell-substance in the process of fertilization; and I
had been especially led to adopt this view because my investigations
upon _Daphnidae_ had shown that an animal produces large sperm-cells
with an immense cell-body whenever the economy of its organism permits.
All _Daphnidae_ in which internal fertilization takes place (in which
the sperm-cells are directly discharged upon the unfertilized egg),
produce a small number of such large sperm-cells (_Sida_, _Polyphemus_,
_Bythotrephes_); while all species with external fertilization
(_Daphnidae_, _Lynceinae_) produce very small sperm-cells in enormous
numbers, thus making up for the immense chances against any single cell
being able to reach an egg. Hence the smaller the chances of any single
sperm-cell being successful, the larger is the number of such cells
produced, and a direct result of this increase in number is a
diminution in size. But why should the sperm-cells remain or become so
large in the species in which fertilization is internal? The idea
suggests itself that the species in this way gains some advantage,
which must be given up in the other cases; although such advantage
might consist in assisting the development of the fertilized ovum and
not in any increase of the true fertilizing substance. At the present
time we are indeed disposed to recognize this advantage in still more
unimportant matters, but at that time the ascertained facts did not
justify us in the assertion that fertilization is a mere fusion of
nuclei, and M. Nussbaum[109] quite correctly expressed the state of our
knowledge when he said that the act of fertilization consisted in ‘the
union of identical parts of two homologous cells.’

Pflüger’s discovery of the ‘isotropism’ of the ovum was the first fact
which distinctly pointed to the conclusion that the bodies of the
germ-cells have no share in the transmission of hereditary tendencies.
He showed that segmentation can be started in different parts of the
body of the egg, if the latter be permanently removed from its natural
position. This discovery constituted an important proof that the body
of the egg consists of a uniform substance, and that certain parts or
organs of the embryo cannot be potentially contained in certain parts
of the egg, so that they can only arise from these respective parts and
from no others. Pflüger was mistaken in the further interpretation,
from which he concluded that the fertilized ovum has no essential
relation to the organization of the animal subsequently formed by it,
and that it is only the recurrence of the same external conditions
which causes the germ-cell to develope always in the same manner. The
force of gravity was the first factor, which, as Pflüger thought,
determined the building up of the embryo: but he overlooked the fact
that isotropism can only be referred to the body of the egg, and that
besides this cell-body there is also a nucleus present, from which it
was at least possible that regulative influences might emanate. Upon
this point Born[110] first showed that the position of the nucleus is
changed in eggs which are thus placed in unnatural conditions, and he
proved that the nucleus must contain a principle which in the first
place directs the formation of the embryo. Roux[111] further showed
that, even when the effect of gravity is compensated, the development
is continued unchanged, and he therefore concluded that the fertilized
egg contains within itself all the forces necessary for normal
development. Finally, O. Hertwig[112] proved from observations on the
eggs of sea-urchins, that at any rate in these animals, gravity has no
directive influence upon segmentation, but that the position of the
first nuclear spindle decides the direction which will be taken by the
first divisional plane of segmentation. These observations were however
still insufficient to prove that fertilization is nothing more than the
fusion of nuclei[113].

A further and more important step was taken when E. van Beneden[114]
observed the process of fertilization in _Ascaris megalocephala_. Like
the investigations of Nussbaum[115] upon the same subject, published at
a rather earlier date, van Beneden’s observations did not altogether
exclude the possibility of the participation of the body of the
sperm-cell in the real process of fertilization; still the fact that
the nuclei of the egg-cell and the sperm-cell do not coalesce
irregularly, but that their loops are placed regularly opposite one
another in pairs and thus form one new nucleus (the first segmentation
nucleus), distinctly pointed to the conclusion that the nuclear
substance is the sole bearer of hereditary tendencies—that in fact
fertilization depends upon the coalescence of nuclei. Van Beneden
himself did not indeed arrive at these conclusions: he was prepossessed
with the idea that fertilization depends upon the union of two sexually
differentiated nuclei, or rather half-nuclei—the male and female
pronuclei. He considered that only in this way could a single complete
nucleus be formed, a nucleus which must of course be hermaphrodite, and
he believed that the essential cause of further development lies in the
fact that, at each successive division of nuclei and cells, this
hermaphrodite nature of the nucleus is maintained by the longitudinal
division of the loops of each mother-nucleus, causing a uniform
distribution of the male and female loops in both daughter-nuclei.

But van Beneden undoubtedly deserves great credit for having
constructed the foundation upon which a scientific theory of heredity
could be built. It was only necessary to replace the terms male and
female pronuclei, by the terms nuclear substance of the male and female
parents, in order to gain a starting-point from which further advance
became possible. This step was taken by Strasburger, who at the same
time brought forward an instance in which the nucleus only of the male
germ-cell (to the exclusion of its cell-body) reaches the egg-cell. He
succeeded in explaining the process of fertilization in Phanerogams,
which had been for a long time involved in obscurity, for he proved
that the nucleus of the sperm-cell (the pollen-tube) enters the
embryo-sac and fuses with the nucleus of the egg-cell: at the same time
he came to the conclusion that the body of the sperm-cell does not pass
into the embryo-sac, so that in this case fertilization can only depend
upon the fusion of nuclei[116].

Thus the nuclear substance must be the sole bearer of hereditary
tendencies, and the facts ascertained by van Beneden in the case of
_Ascaris_ plainly show that the nuclear substance must not only contain
the tendencies of growth of the parents, but also those of a very large
number of ancestors. Each of the two nuclei which unite in
fertilization must contain the germ-nucleoplasm of both parents, and
this latter nucleoplasm once contained and still contains the
germ-nucleoplasm of the grandparents as well as that of all previous
generations. It is obvious that the nucleoplasm of each antecedent
generation must be represented in any germ-nucleus in an amount which
becomes less as the number of intervening generations becomes greater;
and the proportion can be calculated after the manner in which
breeders, when crossing races, determine the proportion of pure blood
which is contained in any of the descendants. Thus while the germ-plasm
of the father or mother constitutes half the nucleus of any fertilized
ovum, that of a grandparent only forms a quarter, and that of the tenth
generation backwards only 1/1024, and so on. The latter can,
nevertheless, exercise influence over the development of the offspring,
for the phenomena of atavism show that the germ-plasm of very remote
ancestors can occasionally make itself felt, in the sudden reappearance
of long-lost characters. Although we are unable to give a detailed
account of the way in which atavism happens, and of the circumstances
under which it takes place, we are at least able to understand how it
becomes possible; for even a very minute trace of a specific germ-plasm
possesses the definite tendency to build up a certain organism, and
will develope this tendency as soon as its nutrition is, for some
reason, favoured above that of the other kinds of germ-plasm present in
the nucleus. Under these circumstances it will increase more rapidly
than the other kinds, and it is readily conceivable that a
preponderance in the quantity of one kind of nucleoplasm may determine
its influence upon the cell-body.

Strasburger—supported by van Beneden’s observations, but in opposition
to the opinions of the latter—had already explained, in a manner
similar to that described above, the process by which the hereditary
transmission of certain characters takes place, and to this extent our
opinions coincide. The nature of heredity is based upon the
transmission of nuclear substance with a specific molecular
constitution. This substance is the specific nucleoplasm of the
germ-cell, to which I have given the name of germ-plasm.

O. Hertwig[117] has also come to the same conclusion: at an earlier
date he had looked upon the coalescence of nuclei as the most essential
feature in the process of fertilization. He now believes that this
former opinion has been confirmed by the recent discoveries which have
been shortly described above.

Although I entirely agree with Hertwig, as far as the main question is
concerned, I cannot share his opinions when he identifies Nägeli’s
idioplasm with the nucleoplasm of the germ-cell. Nägeli’s idioplasm
certainly includes the germ-plasm, if I may retain this expression for
the sake of brevity. Nägeli in forming his hypothesis did indeed start
with the germ-cells, but his idioplasm not only represents the
nucleoplasm of the germ-cells, but also that of all the other cells of
the organism; all these nucleoplasms taken together constitute Nägeli’s
idioplasm. According to Nägeli, the idioplasm forms a network which
extends through the whole body, and represents the specific molecular
basis which determines its nature. Although this latter suggestion—the
general part of his theory—is certainly valid, and although it is of
great importance to have originated the idea of idioplasm in this
general sense, in contrast to the somato-plasm (‘Nährplasma’), it is
nevertheless true that we are not justified in retaining the details of
his theory.

In the first place the idioplasm does not form a directly continuous
network throughout the entire body; and, secondly, the whole organism
is not penetrated by a single substance of homogeneous constitution,
but each special kind of cell must contain the specific idioplasm or
nucleoplasm which determines its nature. There are therefore in each
organism a multitude of different kinds of idioplasm. Thus we should be
quite justified in generally speaking of Nägeli’s idioplasm as
nucleoplasm, and _vice versa_.

It is perfectly certain that the idioplasm cannot form a continuous
network through the whole organism, if it is seated in the nucleus and
not in the cell-body. Even if the bodies of cells are everywhere
connected by fine processes (as has been proved in animals by Leydig
and Heitzmann, and in plants by various botanists), they do not form a
network of idioplasm but of somato-plasm; a substance which, according
to Nägeli, stands in marked contrast to idioplasm. Strasburger has
indeed already spoken of a ‘cyto-idioplasm,’ and it is certainly
obvious that the cell-body often possesses a specific character, but we
must in all cases assume that such a character is impressed upon it by
the influence of the nucleus, or, in other words, that the direction in
which the cell-substance is differentiated in the course of development
is determined by the quality of its nuclear substance. So far,
therefore, the determining nuclear substance corresponds to the
idioplasm alone, while the substance of the cell-body must be
identified with the somato-plasm (‘Nährplasma’) of Nägeli. At all
events, in practice, it will be well to restrict the term idioplasm to
the regulative nuclear substance alone, if we desire to retain the
well-chosen terms of Nägeli’s theory.

But the second part of Nägeli’s theory of the idioplasm is also
untenable. It is impossible that this substance can have the same
constitution everywhere in the organism and during every stage of its
ontogeny. If this were so, how could the idioplasm effect the great
differences which obtain in the formation of the various parts of the
organism? In some passages of his work Nägeli seems to express the same
opinion; e. g. on page 31 he says, ‘It would be practicable to
regard—although only in a metaphorical sense—the idioplasms of the
different cells of an individual as themselves different, inasmuch as
they possess specific powers of production: we should thus include
among these idioplasms all the conditions of the organism which bring
about the display of specific activity on the part of cells.’ It can be
clearly seen from the passages immediately preceding and succeeding the
above-quoted sentence, that Nägeli, in speaking of these changes in the
idioplasm, does not refer to material, but only to dynamical changes.
On page 53 he lays special stress upon the statement that ‘the
idioplasm during its growth retains its specific constitution
everywhere throughout the organism,’ and it is only ‘within these fixed
structural limits that it changes its conditions of tension and
movement, and thus alters the forms of growth and activity which are
possible at each time and place.’ Against such an interpretation
weighty objections can be raised. At present I will only mention that
the meaning of the phrase ‘conditions of tension and movement’ ought to
be made clear, and that we ought to be informed how it is that mere
differences in tension can produce as many different effects as could
have been produced by differences of constitution. If any one were to
assert that in _Daphnidae_, or in any other forms which produce two
kinds of eggs, the power of developing only after a period of rest,
possessed by the winter-eggs, is based upon the fact that their
idioplasm is identical with that of the summer-eggs, but is in another
condition of tension, I should think such a hypothesis would be well
worth consideration, for the animals which arise from the winter-eggs
are identical with those produced in summer: the idioplasm which caused
their formation must therefore be identical in its constitution; and
can only differ in the two cases, as water differs from ice. But the
case is quite otherwise in the stages of ontogeny. How many different
conditions of tension ought to be possessed by one and the same
idioplasm in order to correspond to the thousand different structures
and differentiations of cells in one of the higher organisms? In fact
it would be hardly possible to form even an approximate conception of
an explanation based upon mere ‘conditions of tensions and movement.’
But, furthermore, difference in effect should correspond, at any rate
to some extent, with difference in cause: thus the idioplasm of a
muscle-cell ought to differ more from that of a nerve-cell and of a
digestive-cell in the same individual, than the idioplasm of the
germ-cell of one individual differs from that of other individuals of
the same species; and yet, according to Nägeli, the latter small
difference in the effect is supposed to be due to difference of quality
in the cause—the idioplasm, while the former fundamental difference in
the histological differentiation of cells is supposed to follow from
mere difference ‘of tension and movement.’

Nägeli’s hypothesis appears to be self-contradictory; for, although its
author recognizes the truth of the fundamental law of development, and
explains the stages of ontogeny as an abbreviated recapitulation of
phyletic stages, he nevertheless explains the latter by a different
principle from that which he employs to explain the former. According
to Nägeli, the stages of phylogeny are based upon true qualitative
differences in the idioplasm: the germ-plasm of a worm is qualitatively
different from that of _Amphioxus_, a frog, or a mammal. But if such
phyletic stages occur crowded together in the ontogeny of a single
species, they are said to be based upon different ‘conditions of
tension and movement’ of one and the same idioplasm! It seems to me to
be necessary to conclude that if the idioplasm, in the course of
phyletic development, undergoes any alteration in specific
constitution, such alterations must also take place in ontogeny; so far
at least as the phyletic stages are repeated. Either the whole phyletic
development is based upon different ‘conditions of tension and
movement,’ or if this—as I believe—is impossible, the stages of
ontogeny must be based upon qualitative alterations in the idioplasm.

Involuntarily the question arises—how is it that such an acute thinker
fails to perceive this contradiction? But the answer is not far to
seek, and Nägeli himself indicates it when he adds these words to the
sentence quoted above: ‘It follows therefore that if a cell is detached
as a germ-cell in any stage of ontogenetic development, and from any
part of the organism, such a cell will contain all the hereditary
tendencies of the parent individual.’ In other words, if we are
restricted to different ‘conditions of tension and movement’ as an
explanation, it seems to follow as a matter of course that the
idioplasm can re-assume its original condition, and therefore that the
idioplasm of any cell in the body can again become the idioplasm of the
germ-cell; for this to take place it is only necessary that the greater
tension should become the less, or _vice versa_. But if we admit a real
change in constitution, then the backward development of the idioplasm
of the cells of the body into germ-cells appears to be very far from a
matter of course, and he who assumes it must bring forward weighty
reasons. Nägeli does not produce such reasons, but considers the
metamorphosis of the idioplasm in ontogeny as mere differences in the
‘conditions of tension and movement.’ This phrase covers the weak part
of his theory; and I look upon it as a valuable proof that Nägeli has
also felt that the phenomena of heredity can only find their
explanation in the hypothesis of the continuity of the germ-plasm; for
his phrase is only capable of obscuring the question as to how the
idioplasm of the cells of the body can be re-transformed into the
idioplasm of germ-cells.

I am of the opinion that the idioplasm cannot be re-transformed, and
I have defended this opinion for some years past[118], although I
have hitherto laid especial stress on the positive aspect of the
question, viz. on the continuity of the germ-plasm. I have attempted
to prove that the germ-cells of an organism derive their essential
nature from the fact that the germ-plasm of each generation is
carried over into that which succeeds it; and I have tried to show
that during the development of an egg into an animal, a part of the
germ-substance—although only a minute part—passes over unchanged into
the organism which is undergoing development, and that this part
represents the basis from which future germ-cells arise. In this way
it is to a certain extent possible to conceive how it is that the
complex molecular structure of the germ-plasm can be retained
unchanged, even in its most minute details, through a long series of
generations.

But how would this be possible if the germ-plasm were formed anew in
each individual by the transformation of somatic idioplasm? And yet if
we reject the ‘continuity of the germ-plasm’ we are compelled to adopt
this latter hypothesis concerning its origin. It is the hypothesis
adopted by Strasburger, and we have therefore to consider how the
subject presents itself from his point of view.

I entirely agree with Strasburger when he says, ‘The specific qualities
of organisms are based upon nuclei’; and I further agree with him in
many of his ideas as to the relation between the nucleus and cell-body:
‘Molecular stimuli proceed from the nucleus into the surrounding
cytoplasm; stimuli which, on the one hand, control the phenomena of
assimilation in the cell, and, on the other hand, give to the growth of
the cytoplasm, which depends upon nutrition, a certain character
peculiar to the species.’ ‘The nutritive cytoplasm assimilates, while
the nucleus controls the assimilation, and hence the substances
assimilated possess a certain constitution and nourish in a certain
manner the cyto-idioplasm and the nuclear idioplasm. In this way the
cytoplasm takes part in the phenomena of construction, upon which the
specific form of the organism depends. This constructive activity of
the cyto-idioplasm depends upon the regulative influence of the
nuclei.’ The nuclei therefore ‘determine the specific direction in
which an organism developes.’

The opinion—derived from the recent study of the phenomena of
fertilization—that the nucleus impresses its specific character upon
the cell, has received conclusive and important confirmation in the
experiments upon the regeneration of Infusoria, conducted
simultaneously by M. Nussbaum[119] at Bonn, and by A. Gruber[120] at
Freiburg. Nussbaum’s statement that an artificially separated portion
of a _Paramaecium_, which does not contain any nuclear substance,
immediately dies, must not be accepted as of general application, for
Gruber has kept similar fragments of other Infusoria alive for several
days. Moreover, Gruber had previously shown that individual Protozoa
occur, which live in a normal manner, and are yet without a nucleus,
although this structure is present in other individuals of the same
species. But the meaning of the nucleus is made clear by the fact,
published by Gruber, that such artificially separated fragments of
Infusoria are incapable of regeneration, while on the other hand those
fragments which contain nuclei always regenerate. It is therefore only
under the influence of the nucleus that the cell substance re-developes
into the full type of the species. In adopting the view that the
nucleus is the factor which determines the specific nature of the cell,
we stand on a firm foundation upon which we can build with security.

If therefore the first segmentation nucleus contains, in its molecular
structure, the whole of the inherited tendencies of development, it
must follow that during segmentation and subsequent cell-division, the
nucleoplasm will enter upon definite and varied changes which must
cause the differences appearing in the cells which are produced; for
identical cell-bodies depend, _ceteris paribus_, upon identical
nucleoplasm, and conversely different cells depend upon differences in
the nucleoplasm. The fact that the embryo grows more strongly in one
direction than in another, that its cell-layers are of different nature
and are ultimately differentiated into various organs and
tissues,—forces us to accept the conclusion that the nuclear substance
has also been changed in nature, and that such changes take place
during ontogenetic development in a regular and definite manner. This
view is also held by Strasburger, and it must be the opinion of all who
seek to derive the development of inherited tendencies from the
molecular structure of the germ-plasm, instead of from preformed
gemmules.

We are thus led to the important question as to the forces by which the
determining substance or nucleoplasm is changed, and as to the manner
in which it changes during the course of ontogeny, and on the answer to
this question our further conclusions must depend. The simplest
hypothesis would be to suppose that, at each division of the nucleus,
its specific substance divides into two halves of unequal quality, so
that the cell-bodies would also be transformed; for we have seen that
the character of a cell is determined by that of its nucleus. Thus in
any Metazoon the first two segmentation spheres would be transformed in
such a manner that one only contained the hereditary tendencies of the
endoderm and the other those of the ectoderm, and therefore, at a later
stage, the cells of the endoderm would arise from the one and those of
the ectoderm from the other; and this is actually known to occur. In
the course of further division the nucleoplasm of the first ectoderm
cell would again divide unequally, e.g. into the nucleoplasm containing
the hereditary tendencies of the nervous system, and into that
containing the tendencies of the external skin. But even then, the end
of the unequal division of nuclei would not have been nearly reached;
for, in the formation of the nervous system, the nuclear substance
which contains the hereditary tendencies of the sense-organs, would, in
the course of further cell-division, be separated from that which
contains the tendencies of the central organs, and the same process
would continue in the formation of all single organs, and in the final
development of the most minute histological elements. This process
would take place in a definitely ordered course, exactly as it has
taken place throughout a very long series of ancestors; and the
determining and directing factor is simply and solely the nuclear
substance, the nucleoplasm, which possesses such a molecular structure
in the germ-cell that all such succeeding stages of its molecular
structure in future nuclei must necessarily arise from it, as soon as
the requisite external conditions are present. This is almost the same
conception of ontogenetic development as that which has been held by
embryologists who have not accepted the doctrine of evolution: for we
have only to transfer the primary cause of development, from an unknown
source within the organism, into the nuclear substance, in order to
make the views identical.

It appears at first sight that the knowledge which has been gained by
studying the indirect division of nuclei is opposed to such a view, for
we know that each mother-loop of the so-called nuclear plate divides
longitudinally into two exactly equal halves, which can be stained and
thus rendered visible.

In this way each resulting daughter-nucleus receives an equal supply of
halves, and it therefore appears that the two nuclei must be completely
identical. This at least is Strasburger’s conclusion, and he regards
such identity as a fundamental fact, which cannot be shaken, and with
which all attempts at further explanation must be brought into accord.

How then can the gradual transformation of the nuclear substance be
brought about? For such a transformation must necessarily take place if
the nuclear substance is really the determining factor in development.
Strasburger attempts to support his hypothesis by assuming that the
inequality of the daughter-nuclei arises from unequal nutrition; and he
therefore considers that the inequality is brought about after the
division of the nucleus and of the cell. Strasburger has shown, in a
manner which is above all criticism, that the nucleus derives its
nutrition from the cell-body, but then the cell-bodies of the two _ex
hypothesi_ identical daughter-nuclei must be different from the first,
if they are to influence their nuclei in different ways. But if the
nucleus determines the nature of the cell, it follows that two
identical daughter-nuclei which have arisen by division within one
mother-cell cannot come to possess unequal cell-bodies. As a matter of
fact, however, the cell-bodies of two daughter-cells often differ in
size, in appearance, and in their subsequent history, and these facts
are sufficient to prove that in such cases the division of the nucleus
must have been unequal. It appears to me to be a necessary conclusion
that, in such an instance, the mother-nucleus must have been capable of
splitting into nuclear substances of differing quality. I think that,
in his argument, Strasburger has over-estimated the support afforded by
exact observations upon indirect nuclear division. Certainly the fact,
discovered by Flemming, and more exactly studied by Balbiani and
Pfitzner, that, in nuclear division, the loops split longitudinally, is
of great and even of fundamental importance. Furthermore, the
observations, conducted last year by van Beneden, on the process of
fertilization in _Ascaris_, have given to Flemming’s discovery a
clearer and more definite meaning than could have been at first
ascribed to it. The discovery proves, in the first place, that the
nucleus always divides into two parts of equal quantity, and further
that in every nuclear division, each daughter-nucleus receives the same
amount of nuclear substance from the father as from the mother; but, as
it seems to me, it is very far from proving that the quality of the
parent nucleoplasms must always be equal in the daughter-nuclei. It is
true that the fact seems to prove this; and if we remember the
description of the most favourable instance which has been hitherto
discovered, viz. the process of fertilization in the egg of _Ascaris_,
as represented by van Beneden, the two longitudinal halves of each loop
certainly impress the reader as being absolutely identical (compare,
for instance, loc. cit. Plate XIX, figs. 1, 4, 5). But we must not
forget that we do not see the molecular structure of the nucleoplasm,
but something which we can only look upon (when we remember how complex
this molecular structure must be) as a very rough expression of its
quantity. Our most powerful and best lenses just enable us to make out
the form of separate stainable granules present in a loop which is
about to divide: they appear as spheres and immediately after division
as hemispheres. But according to Strasburger, these granules, the
so-called microsomata, only serve for the nutrition of the nuclear
substance proper, which lies between them unstainable, and therefore
not distinctly visible. But even if these granules represent the true
idioplasm, their division into two exactly equal parts would give us no
proof of equality or inequality in their constitution: it would only
give us an idea of their quantitative relations. We can only obtain
proofs as to the quality of the molecular structure of the two halves
by their effect on the bodies of the daughter-cells, and we know that
these latter are frequently different in size and quality.

This point is so important that I must illustrate it by a few more
examples. The so-called polar bodies (to be treated more in detail
below) which are expelled during maturation from the eggs of so many
animals, are true cells, as was first proved by Bütschli in Nematodes:
their formation is due to a process of undoubted cell-division usually
accompanied by a typical form of indirect nuclear division[121]. If any
one is still in doubt upon this point, after the observations of Fol
and Hertwig, he might easily be convinced of its truth by a glance at
the figures (unfortunately too little known) which Trinchese[122] has
published, illustrating this process in the eggs of certain gastropods.
The eggs of _Amphorina coerulea_ are in every way suitable for
observation, being entirely translucent, and having large distinct
nuclei which differ from the green cytoplasm in colour. In these eggs
two polar bodies are formed one after the other: and each of them
immediately re-divides: hence it follows that four polar bodies are
placed at the pole of the egg. But how is it that these four cells
perish, while the nucleus, remaining in the yolk and conjugating with
the sperm-nucleus, makes use of the whole body of the egg and developes
into the embryo? Obviously because the nature of the polar body is
different from that of the egg-cell. But since the nature of the cell
is determined by the quality of the nucleus, this quality must differ
from the very moment of nuclear division. This is proved by the fact
that the supernumerary spermatozoa which sometimes enter the egg do not
conjugate with the polar bodies. According to Strasburger’s theory, the
objection might be urged that the different quality of the nuclei is
here caused by the very different quantity of cytoplasm by which they
are surrounded and nourished; but on the one hand the smallness of the
cell-bodies which surround most polar globules must have some
explanation, and this can only be found in the nature of the nucleus;
and on the other hand the quantity of the cell-body which surrounds the
polar globules of _Amphorina_ is, as a matter of fact, somewhat larger
than the sphere of green cytoplasm which surrounds the nucleus of the
egg! The difference between the polar bodies and the egg-cell can thus
only be explained on the supposition that, in the division of the
nuclear spindle, two qualitatively different daughter-nuclei are
produced.

There does not seem to be any objection to the view that the
microsomata of the nuclear loops—assuming that these bodies represent
the idioplasm—are capable of dividing into halves, equal in form and
appearance, but unequal in quality. We know that this very process
takes place in many egg-cells; thus in the egg of the earth-worm the
first two segmentation spheres are equal in size and appearance, and
yet the one forms the endoderm and the other the ectoderm of the embryo.

I therefore believe that we must accept the hypothesis that, in
indirect nuclear division, the formation of unequal halves may take
place quite as readily as the formation of equal halves, and that the
equality or inequality of the subsequently produced daughter-cells must
depend upon that of the nuclei. Thus during ontogeny a gradual
transformation of the nuclear substance takes place, necessarily
imposed upon it, according to certain laws, by its own nature, and such
transformation is accompanied by a gradual change in the character of
the cell-bodies.

It is true that we cannot gain any detailed knowledge of the nature of
these changes in the nuclear substance, but we can very well arrive at
certain general conclusions about them. If we may suppose, with Nägeli,
that the molecular structure of the germ-idioplasm, or according to our
terminology the germ-plasm, becomes more complicated according to the
greater complexity of the organism developed from it, then the
following conclusions will also be accepted,—that the molecular
structure of the nuclear substance is simpler as the differences
between the structures arising from it become less; that therefore the
nuclear substance of the segmentation-cell of the earth-worm, which
potentially contains the whole of the ectoderm, possesses a more
complicated molecular structure than that of a single epidermic cell or
nerve-cell. These conclusions will be admitted when it is remembered
that every detail in the whole organism must be represented in the
germ-plasm by its own special and peculiar arrangement of the groups of
molecules (the micellae of Nägeli), and that the germ-plasm not only
contains the whole of the quantitative and qualitative characters of
the species, but also all individual variations as far as these are
hereditary: for example the small depression in the centre of the chin
noticed in some families. The physical causes of all apparently
unimportant hereditary habits or structures, of hereditary talents, and
other mental peculiarities, must all be contained in the minute
quantity of germ-plasm which is possessed by the nucleus of a
germ-cell;—not indeed as the preformed germs of structure (the gemmules
of pangenesis), but as variations in its molecular constitution; if
this be impossible, such characters could not be inherited. Nägeli has
shown in his work, which is so rich in suggestive ideas, that even in
so minute a space as the thousandth of a cubic millimetre, such an
enormous number (400,000,000) of micellae may be present, that the most
diverse and complicated arrangements become possible. It therefore
follows that the molecular structure of the germ-plasm in the
germ-cells of an individual must be distinguished from that of another
individual by certain differences, although these may be but small; and
it also follows that the germ-plasm of any species must differ from
that of all other species.

These considerations lead us to conclude that the molecular structure
of the germ-plasm in all higher animals must be excessively complex,
and, at the same time, that this complexity must gradually diminish
during ontogeny as the structures still to be formed from any cell, and
therefore represented in the molecular constitution of its nucleoplasm,
become less in number. I do not mean to imply that the nucleoplasm
contains preformed structures which are gradually reduced in number as
they are given off in various directions during the building-up of
organs: I mean that the complexity of the molecular structure decreases
as the potentiality for further development also decreases, such
potentiality being represented in the molecular structure of the
nucleus. The nucleoplasm, which in the grouping of its particles
contains potentially a hundred different modifications of this
substance, must possess far more numerous kinds and far more complex
arrangements of such particles than the nucleoplasm which only contains
a single modification, capable of determining the character of a single
kind of cell. The development of the nucleoplasm during ontogeny may be
to some extent compared to an army composed of corps, which are made up
of divisions, and these of brigades, and so on. The whole army may be
taken to represent the nucleoplasm of the germ-cell: the earliest
cell-division (as into the first cells of the ectoderm and endoderm)
may be represented by the separation of the two corps, similarly formed
but with different duties: and the following cell-divisions by the
successive detachment of divisions, brigades, regiments, battalions,
companies, etc.; and as the groups become simpler so does their sphere
of action become limited. It must be admitted that this metaphor is
imperfect in two respects, first, because the quantity of the
nucleoplasm is not diminished, but only its complexity, and secondly,
because the strength of an army chiefly depends upon its numbers, not
on the complexity of its constitution. And we must also guard against
the supposition that unequal nuclear division simply means a separation
of part of the molecular structure, like the detachment of a regiment
from a brigade. On the contrary, the molecular constitution of the
mother-nucleus is certainly changed during division in such a way that
one or both halves receive a new structure which did not exist before
their formation.

My opinion as to the behaviour of the idioplasm during ontogeny, not
only differs from that of Nägeli, in that the latter maintains that the
idioplasm only undergoes changes in its ‘conditions of tension and
movement,’ but also because he imagines this substance to be composed
of the preformed germs of structures (‘Anlagen’). Nägeli’s views are
obviously bound up with his theory of a continuous network of idioplasm
throughout the whole body; perhaps he would have adopted other
conclusions had he been aware of the fact that the idioplasm must only
be sought for in the nuclei. Nägeli’s views as to ontogeny can be best
seen in the following passages: ‘As soon as ontogenetic development
begins, the groups of micellae in the idioplasm which effect the first
stage of development, enter upon active growth: such activity causes a
passive growth of the other groups, and an increase in the whole
idioplasm, perhaps to many times its former bulk. But the intensities
of growth in the two series of groups are unequal, and consequently an
increasing tension is produced which sooner or later, according to the
number, arrangement, and energy of the active groups, necessarily
renders the continuation of the process impossible. In consequence of
such disturbance to the equilibrium, active growth now takes place in
the next group, leading to fresh irritation, and this group then reacts
more strongly than all the others upon the tension which first
stimulated its activity. These changes are repeated until all the
groups are gone through, and the ontogenetic development finally
reaches the stage at which propagation takes place, and thus the
original stage of the germ is reached.’

Hence, according to Nägeli, the different stages of ontogeny arise out
of the activities of different parts of the idioplasm: certain groups
of micellae in the idioplasm represent the germs (‘Anlagen’) of certain
structures in the organism: when any such germ reacts under stimulation
it produces the corresponding structure. It seems to me that this
hypothesis bears some resemblance to Darwin’s theory of pangenesis. I
think that Nägeli’s preformed germs of structures (‘Anlagen’) and his
groups of such germs are highly elaborated equivalents of the gemmules
of pangenesis, which, according to Darwin, manifest activity when their
turn comes, or, according to Nägeli, when they react under stimulation.
When a group of such germs, by their active growth or by their
‘irritation,’ have caused a similar active growth or a similar
irritation in the next group, the former may come to rest, or may
remain in a state of activity together with its successor, for a longer
or shorter period. Its activity may even last for an unlimited time, as
is the case in the formation of leafy shoots in many plants.

Here, again, we recognize the fact that Nägeli’s whole hypothesis is
intimately connected with the supposition that the entire mass of
idioplasm is continuous throughout the organism. Sometimes one part of
the idioplasm and sometimes another part is irritated, and then
produces the corresponding organ. But if, on the other hand, the
idioplasm does not represent a directly continuous mass, but is
composed of thousands of single nucleoplasms which only act together
through the medium of their cell-bodies, then we must substitute the
conception of ‘ontogenetic stages of development of the idioplasm’ for
the conception of germs of structure (‘Anlagen’). The different
varieties of nucleoplasm which arise during ontogeny represent, as it
were, the germs of Nägeli (‘Anlagen’), because, by means of their
molecular structure, they create a specific constitution in the
cell-bodies over which they have control, and also because they
determine the succession of future nuclei and cells.

It is in this sense, and no other, that I can speak of the presence of
preformed germs (‘Anlagen’) in the idioplasm. We may suppose that the
idioplasm of the first segmentation nucleus is but slightly different
from that of the second ontogenetic stage, viz. that of the two
following segmentation nuclei. Perhaps only a few groups of micellae
have been displaced or somewhat differently arranged. But nevertheless
such groups of micellae were not the germs (‘Anlagen’) of a second
stage which pre-existed in the first stage, for the two are
distinguished by the possession of a different molecular structure.
This structure in the second stage, under normal conditions of
development, again brings about the change by which the different
molecular structure of the third stage is produced, and so on.

It may be argued that von Baer’s well-known and fundamental law of
development is opposed to the hypothesis that the idioplasm of
successive ontogenetic stages must gradually assume a simpler molecular
structure. The organization of the species has, on the whole, increased
immensely in complexity during the course of phylogeny: and if the
phyletic stages are repeated in the ontogeny, it seems to follow that
the structure of the idioplasm must become more complex in the course
of ontogeny instead of becoming simpler. But the complexity of the
whole organism is not represented in the molecular structure of the
idioplasm of any single nucleus, but by that of all the nuclei present
at any one time. It is true that the germ-cell, or rather the idioplasm
of the germ-nucleus, must gain greater complexity as the organism which
arises from it becomes more complex; but the individual nucleoplasms of
each ontogenetic stage may become simpler, while the whole mass of
idioplasms in the organism (which, taken together, represent the stage
in question) does not by any means lose in complexity.

If we must therefore assume that the molecular structure of the
nucleoplasm becomes simpler in the course of ontogeny, as the number of
structures which it potentially contains become smaller, it follows
that the nucleoplasm in the cells of fully differentiated tissues—such
as muscle, nerve, sense-organs, or glands—must possess relatively the
most simple molecular structure; for it cannot originate any fresh
modification of nucleoplasm, but can only continue to produce cells of
the same structure, although it does not always retain this power.

We are thus brought back to the fundamental question as to how the
germ-cells arise in the organism. Is it possible that the nucleoplasm
of the germ-cell, with its immensely complex molecular structure,
potentially containing all the specific peculiarities of an individual,
can arise from the nucleoplasm of any of the body-cells,—a substance
which, as we have just seen, has lost the power of originating any new
kind of cell, because of the continual simplification of its structure
during development? It seems to me that it would be impossible for the
simple nucleoplasm of the somatic cells to thus suddenly acquire the
power of originating the most complex nucleoplasm from which alone the
entire organism can be built up: I cannot see any evidence for the
existence of a force which could effect such a transformation.

This difficulty has already been appreciated by other writers.
Nussbaum’s[123] theoretical views, which I have already mentioned, also
depend upon the hypothesis that cells which have once become
differentiated for the performance of special functions cannot be
re-transformed into sexual cells: he also concludes that the latter are
separated from all other cells at a very early period of embryonic
development, before any histological differentiation has taken place.
Valaoritis[124] has also recognised that the transformation of
histologically differentiated cells into sexual cells is impossible. He
was led to believe that the sexual cells of Vertebrata arise from the
white blood corpuscles, for he looked upon these latter as
differentiated to the smallest extent possible. Neither of these views
can be maintained. The former, because the sexual cells of all plants
and most animals are not, as a matter of fact, separated from the
somatic cells at the beginning of ontogeny; the latter, because it is
contradicted by the fact that the sexual cells of vertebrates do not
arise from blood corpuscles, but from the germinal epithelium. But even
if this fact had not been ascertained we should be compelled to reject
Valaoritis’ hypothesis on theoretical grounds, for it is an error to
assume that white blood corpuscles are undifferentiated, and that their
nucleoplasm is similar to the germ-plasm. There is no nucleoplasm like
that of the germ-cell in any of the somatic cells, and no one of these
latter can be said to be undifferentiated. All somatic cells possess a
certain degree of differentiation, which may be rigidly limited to one
single direction, or may take place in one of many directions. All
these cells are widely different from the egg-cell from which they
originated: they are all separated from it by many generations of
cells, and this fact implies that their idioplasms possess a widely
different structure from the idioplasm, or germ-plasm, of the egg-cell.
Even the nuclei of the two first segmentation spheres cannot possess
the same idioplasm as that of the first segmentation nucleus, and it
is, of course, far less possible for such an idioplasm to be present in
the nucleus of any of the later cells of the embryo. The structure of
the idioplasm must necessarily become more and more different from that
of the first segmentation nucleus, as the development of the embryo
proceeds. The idioplasm of the first segmentation nucleus, and of this
nucleus alone, is germ-plasm, and possesses a structure such that an
entire organism can be produced from it. Many writers appear to
consider it a matter of course that any embryonic cell can reproduce
the entire organism, if placed under suitable conditions. But, when we
carefully look into the subject, we see that such powers are not even
possessed by those cells of the embryo which are nearest to the
egg-cell—viz. the first two segmentation spheres. We have only to
remember the numerous cases in which one of them forms the ectoderm of
the animal while the other produces the endoderm, in order to admit the
validity of this objection.

But if the first segmentation spheres are not able to develope into a
complete organism, how can this be the case with one of the later
embryonic cells, or one of the cells of the fully developed animal
body? It is true that we speak of certain cells as being ‘of embryonic
character,’ and only recently Kölliker[125] has given a list of such
cells, among which he includes osteoblasts, cartilage cells, lymph
corpuscles, and connective tissue corpuscles: but even if these cells
really deserve such a designation, no explanation of the formation of
germ-cells is afforded, for the idioplasm of the latter must be widely
different from that of the former.

It is an error to suppose that we gain any further insight into the
formation of germ-cells by referring to these cells of so-called
‘embryonic character,’ which are contained in the body of the mature
organism. It is of course well known that many cells are characterized
by very sharply defined histological differentiation, while others are
but slightly differentiated; but it is as difficult to imagine that
germ-cells can arise from the latter as from the former. Both classes
of cells contain idioplasm with a structure different from that which
is contained in the germ-cell, and we have no right to assume that any
of them can form germ-cells until it is proved that somatic idioplasm
is capable of undergoing re-transformation into germ-idioplasm.

The same argument applies to the cells of the embryo itself, and it
therefore follows that those instances of early separation of sexual
from somatic cells, upon which I have often insisted as indicating the
continuity of the germ-plasm, do not now appear to be of such
conclusive importance as at the time when we were not sure about the
localization of the idioplasm in the nuclei. In the great majority of
cases the germ-cells are not separated at the beginning of embryonic
development, but only in some one of the later stages. A single
exception is found in the pole-cells (‘Polzellen’) of Diptera, as was
shown many years ago by Robin[126] and myself[127]. These are the first
cells formed in the egg, and according to the later observations of
Metschnikoff[128] and Balbiani[129], they become the sexual glands of
the embryo. Here therefore the germ-plasm maintains a true unbroken
continuity. The nucleus of the egg-cell directly gives rise to the
nuclei of the pole-cells, and there is every reason to believe that the
latter receive unchanged a portion of the idioplasm of the former, and
with it the tendencies of heredity. But in all other cases the
germ-cells arise by division from some of the later embryonic cells,
and as these belong to a more advanced ontogenetic stage in the
development of the idioplasm, we can only conclude that continuity is
maintained, by assuming (as I do) that a small part of the germ-plasm
persists unchanged during the division of the segmentation nucleus and
remains mixed with the idioplasm of a certain series of cells, and that
the formation of true germ-cells is brought about at a certain point in
the series by the appearance of cells in which the germ-plasm becomes
predominant. But if we accept this hypothesis it does not make any
difference, theoretically, whether the germ-plasm becomes predominant
in the third, tenth, hundredth, or millionth generation of cells. It
therefore follows that cases of early separation of the germ-cells
afford no proof of a direct persistence of the parent germ-cells in
those of the offspring; for a cell the offspring of which become partly
somatic and partly germ-cells cannot itself have the characters of a
germ-cell; but it may nevertheless contain germ-idioplasm, and may thus
transfer the substance which forms the basis of heredity from the germ
of the parent to that of the offspring.

If we are unwilling to accept this hypothesis, nothing remains but to
credit the idioplasm of each successive ontogenetic stage with a
capability of re-transformation into the first stage. Strasburger
accepts this view; and he believes that the idioplasm of the nuclei
changes during the course of ontogeny, but returns to the condition
of the first stage of the germ, at its close. But the rule of
probability is against such a suggestion. Suppose, for instance, that
the idioplasm of the germ-cell is characterized by ten different
qualities, each of which may be arranged relatively to the others in
two different ways, then the probability in favour of any given
combination would be represented by the fraction (1/2)^{10} = 1/1024:
that is to say, the re-transformation of somatic idioplasm into
germ-plasm will occur once in 1024 times, and it is therefore
impossible for such re-transformation to become the rule. It is also
obvious that the complex structure of the germ-plasm which
potentially contains, with the likeness of a faithful portrait, the
whole individuality of the parent, cannot be represented by only ten
characters, but that there must be an immensely greater number; it is
also obvious that the possibilities of the arrangement of single
characters must be assumed to be much larger than two; so that we get
the formula (1/_p_), where _p_ represents the possibilities, and _n_
the characters. Thus if _n_ and _p_ are but slightly larger than we
assumed above, the probabilities become so slight as to altogether
exclude the hypothesis of a re-transformation of somatic idioplasm
into germ-plasm.

It may be objected that such re-transformation is much more probable in
the case of those germ-cells which separate early from the somatic
cells. Nothing can in fact be urged against the possibility that the
idioplasm of (e. g.) the third generation of cells may pass back into
the condition of the idioplasm of the germ-cell; although of course the
mere possibility does not prove the fact. But there are not many cases
in which the sexual cells are separated so early as the third
generation: and it is very rare for them to separate at any time during
the true segmentation of the egg. In _Daphnidae_ (_Moina_) separation
occurs in the fifth stage of segmentation[130], and although this is
unusually early it does not happen until the idioplasm has changed its
molecular structure six times. In _Sagitta_[131] the separation does
not take place until the archenteron is being formed, and this is after
several hundred embryonic cells have been produced, and thus after the
germ-plasm has changed its molecular structure ten or more times. But
in most cases, separation takes place at a much later stage; thus in
Hydroids it does not happen until after hundreds or thousands of
cell-generations have been passed through; and the same fact holds in
the higher plants, where the production of germ-cells frequently occurs
at the end of ontogeny. In such cases the probability of a
re-transformation of somatic idioplasm into germ-plasm becomes
infinitely small.

It is true that these considerations only refer to a rapid and sudden
re-transformation of the idioplasm. If it could be proved that
development is not merely in appearance but in reality a cyclical
process, then nothing could be urged against the occurrence of
re-transformation. It has been recently maintained by Minot[132] that
all development is cyclical, but this is obviously incorrect, for
Nägeli has already shown that direct non-cyclical courses of
development exist, or at all events courses in which the earliest
condition is not repeated at the close of development. The phyletic
development of the whole organic world clearly illustrates a
development of the latter kind; for although we may assume that organic
development is not nearly concluded, it is nevertheless safe to predict
that it will never revert to its original starting-point, by backward
development over the same course as that which it has already
traversed. No one can believe that existing Phanerogams will ever, in
the future history of the world, retrace all the stages of phyletic
development in precise inverse order, and thus return to the form of
unicellular Algae or Monera; or that existing placental mammals will
develope into Marsupialia, Monotremata, mammal-like reptiles, and the
lower vertebrate forms, into worms and finally into Monera. But how can
a course of development, which seems to be impossible in phylogeny,
occur as the regular method of ontogeny? And quite apart from the
question of possibility, we have to ask for proofs of the actual
occurrence of cyclical development. Such a proof would be afforded if
it could be shown that the nucleoplasm of those somatic cells which
(e.g. in Hydroids) are transformed into germ-cells passes backwards
through many stages of development into the nucleoplasm of the
germ-cell. It is true that we can only recognise differences in the
structure of the idioplasm by its effects upon the cell-body, but no
effects are produced which indicate that such backward development
takes place. Since the course of onward development is compelled to
pass through the numerous stages which are implied in segmentation and
the subsequent building-up of the embryo, etc., it is quite impossible
to assume that backward development would take place suddenly. It would
be at least necessary to suppose that the cells of embryonic character,
which are said to be transformed into primitive germ-cells, must pass
back through at any rate the main phases of their ontogeny. A sudden
transformation of the nucleoplasm of a somatic cell into that of a
germ-cell would be almost as incredible as the transformation of a
mammal into an amoeba; and yet we are compelled to admit that the
transformation must be sudden, for no trace of such retrogressive
stages of development can be seen. If the appearance of the whole cell
gives us any knowledge as to the structure of its nuclear idioplasm, we
may be sure that the development of a primitive germ-cell proceeds
without a break, from the moment of its first recognizable formation,
to the ultimate production of distinct male or female sexual cells.

I am well aware that Strasburger has stated that, in the ultimate
maturation of the sexual cells, the substance of the nuclei returns to
a condition similar to that which existed at the beginning of
ontogenetic development; still such a statement is no proof, but only
an assumption made to support a theory. I am also aware that Nussbaum
and others believe that, in the formation of spermatozoa in higher
animals, a backward development sets in at a certain stage; but even if
this interpretation be correct, such backward development would only
lead as far as the primitive germ-cell, and would afford no explanation
of the further transformation of the idioplasm of this cell into
germ-plasm. But this latter transformation is just the point which most
needs proof upon any theory except the one which assumes that the
primitive germ-cell still contains unchanged germ-plasm. Every attempt
to render probable such a re-transformation of somatic nucleoplasm into
germ-plasm breaks down before the facts known of the Hydroids, in which
only certain cells in the body, out of the numerous so-called embryonic
cells, are capable of becoming primitive germ-cells, while the rest do
not possess this power.

I must therefore consider as erroneous the hypothesis which assumes
that the somatic nucleoplasm may be transformed into germ-plasm. Such a
view may be called ‘the hypothesis of the cyclical development of the
germ-plasm.’

Nägeli has tried to support such an hypothesis on phyletic grounds. He
believes that phyletic development follows from an extremely slow but
steady change in the idioplasm, in the direction of greater complexity,
and that such changes only become visible periodically. He believes
that the passage from one phyletic stage to another is chiefly due to
the fact that ‘in any ontogeny, the very last structural change upon
which the separation of germs depends, takes place in a higher stage,
one or more cell-generations later’ than it occurred in a lower stage.
‘The last structural change itself remains the same, while the series
of structural changes immediately preceding it is increased.’ I believe
that Nägeli, being a botanist, has been too greatly influenced by the
phenomena of plant-life. It is certainly true that in plants, and
especially in the higher forms, the germ-cells only make their
appearance, as it were, at the end of ontogeny; but facts such as these
do not hold in the animal kingdom: at any rate they are not true in the
great majority of cases. In animals, as I have already mentioned
several times, the germ-cells are separated from the somatic cells
during embryonic development, sometimes even at its very commencement;
and it is obvious that this latter is the original, phyletically
oldest, mode of formation. The facts at our disposal indicate that the
germ-cells only appear, for the first time, after embryological
development, in those cases where the formation of asexually produced
colonies takes place, either with or without alternation of
generations; or in cases where alternation of generations occurs
without the formation of such colonies. In a colony of polypes, the
germ-cells are produced by the later generations, and not by the
founder of the colony which was developed from an egg. This is also
true of the colonies of Siphonophora, and the germ-cells appear to
arise very late in certain instances of protracted metamorphosis
(Echinodermata), but on the other hand, they arise during the embryonic
development of other forms (Insecta) which also undergo metamorphosis.
It is obvious that the phyletic development of colonies or stocks must
have succeeded that of single individuals, and that the formation of
germ-cells in the latter must therefore represent the original method.
Thus the germ-cells originally arose at the beginning of ontogeny and
not at its close, when the somatic cells are formed.

This statement is especially supported by the history of certain lower
plants, or at any rate chlorophyll-containing organisms, and I think
that these forms supply an admirable illustration of my theory as to
the phyletic origin of germ-cells, as explained in my earlier papers
upon the same subject.

The phyletic origin of germ-cells obviously coincides with the
differentiation of the first multicellular organisms by division of
labour[133]. If we desire to investigate the relation between
germ-cells and somatic cells, we must not only consider the highly
developed and strongly differentiated multicellular organisms, but we
must also turn our attention to those simpler forms in which phyletic
transitions are represented. In addition to solitary unicellular
organisms, we know of others living in colonies of which the
constituent units or cells (each of them equivalent to a unicellular
organism) are morphologically and physiologically identical. Each unit
feeds, moves, and under certain circumstances is capable of reproducing
itself, and of thus forming a new colony by repeated division. The
genus _Pandorina_ (Fig. I), belonging to the natural order
_Volvocineae_, represents such ‘homoplastid’ (Götte) organisms. It
forms a spherical colony composed of ciliated cells, all of which are
exactly alike: they are embedded in a colourless gelatinous mass. Each
cell contains chlorophyll, and possesses a red eye-spot, and a
pulsating vacuole. These colonies are propagated by the sexual and
asexual (Fig. II) methods alternately, although in the former case the
conjugating swarm-cells cannot be distinguished with certainty as male
or female. In both kinds of reproduction, each cell in the colony acts
as a reproductive cell; in fact, it behaves exactly like a unicellular
organism.

[Illustration: I. _Pandorina morum_ (after Pringsheim), a swarming
colony. II. A colony divided into sixteen daughter colonies: all the
cells alike. III. A young individual of _Volvox minor_ (after Stein),
still enclosed in the wall of the cell from which it has been
parthenogenetically produced. The constituent cells are divided into
somatic (_sz_), germ-cells (_kz_).]

It is very interesting to find in another genus belonging to the same
natural order, that the transition from the homoplastid to the
heteroplastid condition, and the separation into somatic and
reproductive cells, have taken place. In _Volvox_ (Fig. III) the
spherical colony consists of two kinds of cells, viz. of very numerous
small ciliated cells, and of a much smaller number of large germ-cells
without cilia. The latter alone possess the power of producing a new
colony, and this takes place by the asexual and sexual methods
alternately: in the latter a typical fertilization of large egg-cells
by small spermatozoa occurs. The sexual differentiation of the
germ-cells is not material to the question we are now considering; the
important point is to ascertain whether here, at the very origin of
heteroplastid organisms, the germ-cells, sexually differentiated or
not, arise from the somatic cells _at the end of ontogeny_, or whether
the substance of the parent germ-cell, during embryonic development, is
_from the first_ separated into somatic and germ-cells. The former
interpretation would support Nägeli’s view, the latter would support my
own. But Kirchner[134] distinctly states that the germ-cells of
_Volvox_ are differentiated during embryonic development, that is,
before the escape of the young heteroplastid organism from the
egg-capsule. We cannot therefore imagine that the phyletic development
of the first heteroplastid organism took place in a manner different
from that which I have previously advocated on theoretical grounds,
before this striking instance occurred to me. The germ-plasm
(nucleoplasm) of some homoplastid organism (similar to _Pandorina_)
must have become modified in molecular structure during the course of
phylogeny, so that the colony of cells produced by its division was no
longer made up of identical units, but of two different kinds. After
this separation, the germ-cells alone retained the power of
reproduction possessed by all the parent cells, while the rest only
retained the power of producing similar cells by division. Thus
_Volvox_ seems to afford distinct evidence that in the phyletic origin
of the heteroplastid groups, somatic cells were not, as Nägeli
supposes, intercalated between the mother germ-cell and the daughter
germ-cells in each ontogeny, but that the somatic cells arose directly
from the former, with which they were previously identical, as they are
even now in the case of _Pandorina_. Thus the continuity of the
germ-plasm is established at least for the beginning of the phyletic
series of development.

The fact, already often mentioned, that in most higher organisms the
separation of germ-cells takes place later, and often very late, at the
end of the whole ontogeny, proves that the time at which this
separation of the two kinds of cells took place, must have been
gradually changed. In this respect the well-established instances of
early separation are of great value, because they serve to connect the
extreme cases. It is quite impossible to maintain that the germ-cells
of Hydroids or of the higher plants, exist from the time of embryonic
development, as indifferent cells, which cannot be distinguished from
others, and which are only differentiated at a later period. Such a
view is contradicted by the simplest mathematical consideration; for it
is obvious that none of the relatively few cells of the embryo can be
excluded from the enormous increase by division, which must take place
in order to produce the large number of daughter individuals which form
a colony of polypes. It is therefore clear that all the cells of the
embryo must for a long time act as somatic cells, and none of them can
be reserved as germ-cells and nothing else: this conclusion is moreover
confirmed by direct observation. The sexual bud of a _Coryne_ arises at
a part of the Polype which does not in any way differ from surrounding
areas, the body wall being uniformly made up of two single layers of
cells, the one forming the ectoderm and the other the endoderm. Rapid
growth then takes place at a single spot, and some of the young cells
thus produced are transformed into germ-cells, which did not previously
exist as separate cells.

Strictly speaking I have therefore fallen into an inaccuracy in
maintaining (in former works) that the germ-cells are themselves
immortal; they only contain the undying part of the organism—the
germ-plasm; and although this substance is, as far as we know,
invariably surrounded by a cell-body, it does not always control the
latter, and thus confer upon it the character of a germ-cell. But this
admission does not materially change our view of the whole subject. We
may still contrast the germ-cells, as the undying part of the Metazoan
body, with the perishable somatic cells. If the nature and the
character of a cell is determined by the substance of the nucleus and
not by the cell-body, then the immortality of the germ-cells is
preserved, although only the nuclear substance passes uninterruptedly
from one generation to another.

G. Jäger[135] was the first to state that the body in the higher
organisms is made up of two kinds of cells, viz., ontogenetic and
phyletic cells, and that the latter, the reproductive cells, are not a
product of the former (the body-cells), but that they arise directly
from the parent germ-cell. He assumed that the formation of germ-cells
takes place at the earliest stage of embryonic life, and he thus
believed the connexion between the germ-plasm of the parent and of the
offspring had received a satisfactory explanation. As I have previously
mentioned in the introduction, Nussbaum also brought forward this
hypothesis at a later period, and also based it upon a continuity of
the germ-cells. He assumed that the fertilized egg is divided into the
cells of the individual and into the cells which effect the
preservation of the species, and he supported this view by referring to
the few known cases of early separation of the sexual cells. He even
maintained this hypothesis when I had proved in my investigations on
Hydromedusae that the sexual cells are not always separated from the
somatic cells during embryonic development, but often at a far later
period. Not only is the hypothesis of a direct connexion between the
germ-cells of the offspring and parent broken down by the facts known
in the Hydroids, and in the Phanerogams[136] which resemble them in
this respect, but even the instances of early separated germ-cells
quoted by Jäger and Nussbaum do not as a matter of fact support their
hypothesis. Among existing organisms it is extremely rare for the
germ-cells to arise directly from the parent egg-cell (as in Diptera).
If, however, the germ-cells are separated only a few cell-generations
later, the postulated continuity breaks down; for an embryonic cell, of
which the offspring are partly germ-cells and partly somatic cells,
cannot itself possess the nature of a germ-cell, and its idioplasm
cannot be identical with that of the parent germ-cell. In order to
prove this, it is only necessary to refer to the arguments as to the
ontogenetic stages of the idioplasm. In the above-mentioned instances,
the continuity from the germ-substance of the parent to that of the
offspring can only be explained by the supposition that the somatic
nucleoplasm still contains some unchanged germ-plasm. I believe that
the fundamental idea of Jäger and Nussbaum is quite correct: it is the
same idea which has led me to the hypothesis of the continuity of the
germ-plasm, viz., the conviction that heredity can only be understood
by means of such an hypothesis. But both these writers have worked out
the idea in the form of an hypothesis which does not correspond with
the facts. That this is the case is also shown by the following words
of Nussbaum—‘the cell-material of the individual (somatic cells) can
never produce a single sexual cell.’ Such production undoubtedly takes
place, not only in Hydroids and Phanerogams, but in many other
instances. The germ-cells cannot indeed be produced by any indifferent
cell of embryonic character, but by certain cells, and under
circumstances which allow us to positively conclude that they have been
predestined for this purpose from the beginning. In other words, the
cells in question contain germ-plasm, and this alone enables them to
become germ-cells.

As a result of my investigations on Hydroids[137], I concluded that the
germ-plasm is present in a very finely divided and therefore invisible
state in certain somatic cells, from the very beginning of embryonic
development, and that it is then transmitted through innumerable
cell-generations, to those remote individuals of the colony in which
sexual products are formed. This conclusion is based upon the fact that
germ-cells only occur in certain localized areas (‘Keimstätten’) in
which neither germ-cells nor primitive germ-cells (the cells which are
transformed into germ-cells at a later period) were previously present.
The primitive germ-cells are also only formed in localized areas,
arising from somatic cells of the ectoderm. The place at which
germ-cells arise is the same in all individuals of the same species;
but differs in different species. It can be shown that such differences
correspond to different phyletic stages of a process of displacement,
which tends to remove the localized area from its original position
(the manubrium of the Medusa) in a centripetal direction. For the
purposes of the present enquiry it is unnecessary to discuss the
reasons for this change of position. The phyletic displacements of the
localized areas are brought about during ontogeny by an actual
migration of primitive germ-cells from the place where they arose to
the position at which they undergo differentiation into germ-cells. But
we cannot believe that primitive germ-cells would migrate if the
germ-cells could be formed from any of the other young cells of
indifferent character which are so numerous in Hydroids. Even when the
localized area undergoes very slight displacement, e.g. when it is
removed from the exterior to the interior of the mesogloea[138], the
change is always effected by active migration of primitive germ-cells
through the substance of the mesogloea. Although the localized area has
been largely displaced in the course of phylogeny, the changes in
position have always taken place by very gradual stages, and never
suddenly, and all these stages are repeated in the ontogeny of all
existing species, by the migration of the primitive germ-cells from the
ancestral area to the place where the germ-cells now arise.
Hartlaub[139] has recently added a further instance (that of _Obelia_)
to the numerous minute descriptions of these phyletic displacements of
the localized area, and ontogenetic migrations of the primitive
germ-cells, which are given in my work already referred to. The
instance of _Obelia_ is of especial interest as the direction of
displacement is here reversed, taking place centrifugally instead of in
a centripetal direction.

But if displacements of the localized areas can only take place by the
frequently roundabout method of the migration of primitive germ-cells,
we are obliged to conclude that such is the only manner in which the
change can be effected, and that other cells are unable to play the
role of the primitive germ-cells. And if other cells are unable to take
this part, it must be because nucleoplasm of a certain character has to
be present in order to form germ-cells, or according to the terms of my
theory, the presence of germ-plasm is indispensable for this purpose. I
do not see how we can escape the conclusion that there is continuity of
the germ-plasm; for if it were supposed that somatic idioplasm
undergoes transformation into germ-plasm, such an assumption would not
explain why the displacement occurs by small stages, and with extreme
and constant care for the preservation of a connexion with cells of the
ancestral area. This fact can only be explained by the hypothesis that
cell-generations other than those which end in the production of the
cells of the ancestral area, are totally incapable of transformation
into germ-cells.

Strasburger has objected that the transmission of germ-plasm along
certain lines, viz. through a certain succession of somatic cells, is
impossible, because the idioplasm is situated in the nucleus and not in
the cell-body, and because a nucleus can only divide into two exactly
equal halves by the indirect method of division, which takes place, as
we must believe, in these cases. ‘It might indeed be supposed,’ says
Strasburger, ‘that during nuclear division certain molecular groups
remain unchanged in the nuclear substance which is in other respects
transformed, and that these groups are uniformly distributed through
the whole organism; but we cannot imagine that their transmission could
only be effected along certain lines.’

I do not think that Strasburger’s objections can be maintained. I base
this opinion on my previous criticism upon the assumed equality of the
two daughter-nuclei formed by indirect division. I do not see any
reason why the two halves must always possess the same structure,
although they may be of equal size and weight. I am surprised that
Strasburger should admit the possibility that the germ-plasm, which, as
I think, is mixed with the idioplasm of the somatic cells, may remain
unchanged in its passage through the body; for if this writer be
correct in maintaining that the changes of nuclear substance in
ontogeny are effected by the nutritive influence of the cell-body
(cytoplasm), it follows that the whole nuclear substance of a cell must
be changed at every division, and that no unchanged part can remain. We
can only imagine that one part of a nucleus may undergo change while
the other part remains unchanged, if we hold that the necessary
transformations of nuclear substance are effected, by purely internal
causes, viz. that they follow from the constitution of the nucleoplasm.
But that one part may remain unchanged, and that such persistence does,
as a matter of fact, occur is shown by the cases above described, in
which the germ-cells separate very early from the developing egg-cell.
Thus in the egg of Diptera, the two nuclei which are first separated by
division from the segmentation nucleus, form the sexual cells, and this
proves that they receive the germ-plasm of the segmentation nucleus
unchanged. But during or before the separation of these two nuclei, the
remaining part of the segmentation nucleus must have become changed in
nature, or else it would continue to form ‘pole-cells’ at a later
period instead of forming somatic cells. Although in many cases the
cell-bodies of such early embryonic cells fail to exhibit any visible
differences, the idioplasm of their nuclei must undoubtedly differ, or
else they could not develope in different directions. It seems to me
not only possible, but in every way probable, that the bodies of such
early embryonic cells are equal in reality as well as in appearance;
for, although the idioplasm of the nucleus determines the character of
the cell-body, and although every differentiation of the latter depends
upon a certain structure of its nucleoplasm, it does not necessarily
follow that the converse proposition is true, viz. that each change in
the structure of the nucleoplasm must effect a change in the cell-body.
Just as rain is impossible without clouds, but every cloud does not
necessarily produce rain, so growth is impossible without chemical
change, but chemical processes of every kind and degree need not
produce growth. In the same manner every kind of change in the
molecular structure of the nucleoplasm need not exercise a transforming
influence on the cytoplasm, and we can easily imagine that a long
series of changes in the nucleoplasm may appear only in the kind and
energy of the nuclear divisions which take place, the cell-substance
remaining unchanged, as far as its molecular and chemical structure is
concerned. This suggestion is in accordance with the fact that during
the first period of embryonic development in animals, the cell-bodies
do not exhibit any visible differences, or only such as are very
slight; although exceptional instances occur, especially among the
lower animals. But even these latter (e.g. the difference in appearance
of the cells of the ectoderm and endoderm in sponges and Coelenterata)
perhaps depend more largely upon a different admixture of nutritive
substances than upon any marked difference in the cytoplasm itself. It
is obvious that, in the construction of the embryo, the amount of
cell-material must be first of all increased, and that it is only at a
later period that the material must be differentiated so as to possess
various qualities, according to the principle of division of labour.
Facts of this kind are also opposed to Strasburger’s view, that the
cause of changes in the nucleoplasm does not lie within this substance
itself but within the cell-body.

I believe I have shown that theoretically hardly any objections can be
raised against the view that the nuclear substance of somatic cells may
contain unchanged germ-plasm, or that this germ-plasm may be
transmitted along certain lines. It is true that we might imagine _a
priori_ that all somatic nuclei contain a small amount of unchanged
germ-plasm. In Hydroids such an assumption cannot be made, because only
certain cells in a certain succession possess the power of developing
into germ-cells; but it might well be imagined that in some organisms
it would be a great advantage if every part possessed the power of
growing up into the whole organism and of producing sexual cells under
appropriate circumstances. Such cases might exist if it were possible
for all somatic nuclei to contain a minute fraction of unchanged
germ-plasm. For this reason, Strasburger’s other objection against my
theory also fails to hold; viz. that certain plants can be propagated
by pieces of rhizomes, roots, or even by means of leaves, and that
plants produced in this manner may finally give rise to flowers, fruit
and seeds, from which new plants arise. ‘It is easy to grow new plants
from the leaves of _Begonia_ which have been cut off and merely laid
upon moist sand, and yet in the normal course of ontogeny the molecules
of germ-plasm would not have been compelled to pass through the leaf;
and they ought therefore to be absent from its tissue. Since it is
possible to raise from the leaf a plant which produces flower and
fruit, it is perfectly certain that special cells containing the germ
substance cannot exist in the plant.’ But I think that this fact only
proves, that in _Begonia_ and similar plants, all the cells of the
leaves or perhaps only certain cells contain a small amount of
germ-plasm, and that consequently these plants are specially adapted
for propagation by leaves. How is it then that all plants cannot be
reproduced in this way? No one has ever grown a tree from the leaf of
the lime or oak, or a flowering plant from the leaf of the tulip or
convolvulus. It is insufficient to reply that, in the last-mentioned
cases, the leaves are more strongly specialized, and have thus become
unable to produce germ-substance; for the leaf-cells in these different
plants have hardly undergone histological differentiation in different
degrees. If, notwithstanding, the one can produce a flowering plant,
while the others have not this power, it is of course clear that
reasons other than the degree of histological differentiation must
exist; and, according to my opinion, such a reason is to be found in
the admixture of a minute quantity of unchanged germ-plasm with some of
their nuclei.

In Sachs’ excellent lectures on the physiology of plants, we read on
page 723[140]—‘In the true mosses almost any cell of the roots, leaves
and shoot-axes, and even of the immature sporogonium, may grow out
under favourable conditions, become rooted, form new shoots, and give
rise to an independent living plant.’ Since such plants produce
germ-cells at a later period, we have here a case which requires the
assumption that all or nearly all cells must contain germ-plasm.

The theory of the continuity of the germ-plasm seems to me to be still
less disproved or even rendered improbable by the facts of the
alternation of generations. If the germ-plasm may pass on from the egg
into certain somatic cells of an individual, and if it can be further
transmitted along certain lines, there is no difficulty in supposing
that it may be transmitted through a second, third, or through any
number of individuals produced from the former by budding. In fact, in
the Hydroids, on which my theory of the continuity of the germ-plasm
has been chiefly based, alternation of generations is the most
important means of propagation.

               II. The Significance of the Polar Bodies.

We have already seen that the specific nature of a cell depends upon
the molecular structure of its nucleus; and it follows from this
conclusion that my theory is further, and as I believe strongly,
supported, by the phenomenon of the expulsion of polar bodies, which
has remained inexplicable for so long a time.

For if the specific molecular structure of a cell-body is caused and
determined by the structure of the nucleoplasm, every kind of cell
which is histologically differentiated must have a specific
nucleoplasm. But the egg-cell of most animals, at any rate during the
period of growth, is by no means an indifferent cell of the most
primitive type. At such a period its cell-body has to perform quite
peculiar and specific functions; it has to secrete nutritive substances
of a certain chemical nature and physical constitution, and to store up
this food-material in such a manner that it may be at the disposal of
the embryo during its development. In most cases the egg-cell also
forms membranes which are often characteristic of particular species of
animals. The growing egg-cell is therefore histologically
differentiated: and in this respect resembles a somatic cell. It may
perhaps be compared to a gland-cell, which does not expel its
secretion, but deposits it within its own substance[141]. To perform
such specific functions it requires a specific cell-body, and the
latter depends upon a specific nucleus. It therefore follows that the
growing egg-cell must possess nucleoplasm of specific molecular
structure, which directs the above-mentioned secretory functions of the
cell. The nucleoplasm of histologically differentiated cells may be
called histogenetic nucleoplasm, and the growing egg-cell must contain
such a substance, and even a certain specific modification of it. This
nucleoplasm cannot possibly be the same as that which, at a later
period, causes embryonic development. Such development can only be
produced by true germ-plasm of immensely complex constitution, such as
I have previously attempted to describe. It therefore follows that the
nucleus of the egg-cell contains two kinds of nucleoplasm:—germ-plasm
and a peculiar modification of histogenetic nucleoplasm, which may be
called _ovogenetic nucleoplasm_. This substance must greatly
preponderate in the young egg-cell, for, as we have already seen, it
controls the growth of the latter. The germ-plasm, on the other hand,
can only be present in minute quantity at first, but it must undergo
considerable increase during the growth of the cell. But in order that
the germ-plasm may control the cell-body, or, in other words, in order
that embryonic development may begin, the still preponderating
ovogenetic nucleoplasm must be removed from the cell. This removal
takes place in the same manner as that in which differing nuclear
substances are separated during the ontogeny of the embryo: viz. by
nuclear division, leading to cell-division. The expulsion of the polar
bodies is nothing more than the removal of ovogenetic nucleoplasm from
the egg-cell. That the ovogenetic nucleoplasm continues to greatly
preponderate in the nucleus up to the very last, may be concluded from
the fact that two successive divisions of the latter and the expulsion
of two polar bodies appear to be the rule. If in this way a small part
of the cell-body is expelled from the egg, the extrusion must in all
probability be considered as an inevitable loss, without which the
removal of the ovogenetic nucleoplasm cannot be effected.

This is my theory of the significance of polar bodies, and I do not
intend to contrast it, _in extenso_, with the theories propounded by
others; for such theories are well known and differ essentially from my
own. All writers agree in supposing that something which would be an
obstacle to embryonic development is removed from the egg; but opinions
differ as to the nature of this substance and the precise reasons for
its removal[142]. Some observers (e. g. Minot[143], van Beneden, and
Balfour) regard the nucleus as hermaphrodite, and assume that in the
polar bodies the male element is expelled in order to render the egg
capable of fertilization. Others speak of a rejuvenescence of the
nucleus, others again believe that the quantity of nuclear substance
must be reduced in order to become equal to that of the generally
minute sperm-nucleus, and that the proportions for nuclear conjugation
are in this way adjusted.

The first view seems to me to be disproved by the fact that male as
well as female qualities are transmitted by the egg-cell, while the
sperm-cell also transmits female qualities. The germ-plasm of the
nucleus of the egg cannot therefore be considered as female, and that
of the sperm-nucleus cannot be considered as male: both are sexually
indifferent. The last view has been recently formulated by Strasburger,
who holds that the quantity of the idioplasm contained in the
germ-nucleus must be reduced by one half, and that a whole nucleus is
again produced by conjugation with the sperm-nucleus. Although I
believe that the fundamental idea underlying this hypothesis is
perfectly correct, viz. that the influence of each nucleus is as
largely dependent upon its quantity as upon its quality, I must raise
the objection that the decrease in quantity is not the explanation of
the expulsion of polar bodies. The quantity of idioplasm contained in
the germ-nucleus is, as a matter of fact, not reduced by one-half but
by three-fourths, for two divisions take place one after the other.
Thus by conjugation with the sperm-nucleus, which we may assume to be
of the same size as the germ-nucleus, a nucleus is produced which can
only contain half as much idioplasm as was present in the original
germ-nucleus, before division. Strasburger’s view leaves unexplained
the question why the size of the germ-nucleus, before the expulsion of
polar bodies, was thus twice as large; and even if we neglect the
theory of ovogenetic nucleoplasm and assume that this larger nucleus
was entirely made up of germ-plasm, it must be asked why the egg did
not commence segmentation earlier. The theory which explains the
sperm-cell as the vitalizing principle which starts embryonic
development, like the spark which kindles the gunpowder, would indeed
answer this question in a very simple manner. But Strasburger does not
accept this theory, and holds, as I do, a very different view, which
will be explained later on.

If, on the other hand, we assume that the germ-nucleus contains two
different kinds of nucleoplasm, the question is answered quite
satisfactorily. In treating of parthenogenesis, further on, I shall
mention a fact which seems to me to furnish a real proof of the
validity of this explanation; and, if we accept this fact for the
present, it will be clear that the simple explanation now offered of
phenomena which are otherwise so difficult to understand, would also
largely support the theory of the continuity of the germ-plasm. Such an
explanation would, above all, very clearly demonstrate the co-existence
of two nucleoplasms with different qualities in one and the same
nucleus. My theory must stand or fall with this explanation, for if the
latter were disproved, the continuity of the germ-plasm could not be
assumed in any instance, not even in the simplest cases, where, as in
Diptera, the germ-cells are the first-formed products of embryonic
development. For even in these insects the egg possesses a specific
histological character which must depend upon a specifically
differentiated nucleus. If then two kinds of nucleoplasm are not
present, we must assume that in such cases the germ-plasm of the newly
formed germ-cells, which has passed on unchanged from the segmentation
nucleus, is at once transformed entirely into ovogenetic nucleoplasm,
and must be re-transformed into germ-plasm at a later period when the
egg is fully mature. We could not in any way understand why such a
re-transformation requires the expulsion of part of the nuclear
substance.

At all events, my explanation is simpler than all others, in that it
only assumes a single transformation of part of the germ-plasm, and not
the later re-transformation of ovogenetic nucleoplasm into germ-plasm,
which is so improbable. The ovogenetic nucleoplasm must possess
entirely different qualities from the germ-plasm; and, above all, it
does not readily lead to division, and thus we can better understand
the fact, in itself so remarkable, that egg-cells do not increase in
number by division, when they have assumed their specific structure,
and are controlled by the ovogenetic nucleoplasm. The tendency to
nuclear division, and consequently to cell-division, is not produced
until changes have to a certain extent taken place in the mutual
relation between the two kinds of nucleoplasm contained in the
germ-nucleus. This change is coincident with the attainment of maximum
size by the body of the egg-cell. Strasburger, supported by his
observations on _Spirogyra_, concludes that the stimulus towards
cell-division emanates from the cell-body; but the so-called centres of
attraction at the poles of the nuclear spindle obviously arise under
the influence of the nucleus itself, even if we admit that they are
entirely made up of cytoplasm. But this point has not been decided
upon, and we may presume that the so-called ‘Polkörperchen’ of the
spindle (Fol) are derived from the nucleus, although they are placed
outside the nuclear membrane[144]. Many points connected with this
subject are still in a state of uncertainty, and we must abstain from
general conclusions until it has been possible to demonstrate clearly
the precise nature of certain phenomena attending indirect nuclear
division, which still remain obscure in spite of the efforts of so many
excellent observers. We cannot even form a decided opinion as to
whether the chromatin or the achromatin of the nuclear thread is the
real idioplasm. But although these points are not yet thoroughly
understood, we are justified in maintaining that the cell enters upon
division under the influence of certain conditions of the nucleus,
although the latter are invisible until cell-division has already
commenced.

I now pass on to examine my hypothesis as to the significance of the
formation of polar bodies, in the light of those ascertained facts
which bear upon it.

If the expulsion of the polar bodies means the removal of the
ovogenetic nucleoplasm after the histological differentiation of the
egg-cell is complete, we must expect to find polar bodies in all
species except those in which the egg-cell has remained in a primitive
undifferentiated condition, if indeed such species exist. Wherever the
egg-cell assumes the character of a specialized cell, e.g. in the
attainment of a particular size or constitution, in the admixture of
food-yolk, or the formation of membranes, it must also contain
ovogenetic nucleoplasm, which must ultimately be removed if the
germ-plasm is to gain control over the egg-cell. It does not signify at
all, in this respect, whether the egg is or is not destined for
fertilization.

If we examine the Metazoa in regard to this question, we find that
polar bodies have not yet been discovered in sponges[145], but this
negative evidence is no proof that they are really absent. In all
probability, no one has ever seriously endeavoured to find them, and
there are perhaps difficulties in the way of the proofs of their
existence, because the egg-cell lies free for a long time and even
moves actively in the tissue of the mesogloea. We might expect that the
formation of polar bodies takes place here, as in all other instances,
when the egg becomes mature, that is, at a time when the eggs are
already closely enveloped in the sponge tissue. At all events the eggs
of sponges, as far as they are known, attain a specific nature, in the
possession of a peculiar cell-body, frequently containing food-yolk,
and of the nucleus which is characteristic of all animal eggs during
the process of growth. Hence we cannot doubt the presence of a specific
ovogenetic nucleoplasm, and must therefore also believe that it is
ultimately removed in the polar bodies.

In other Coelenterata, in worms, echinoderms, and in molluscs polar
bodies have been described, as well as in certain Crustacea, viz. in
_Balanus_ by Hoek and in _Cetochilus septentrionale_ by Grobben. The
latter instance appears to be quite trustworthy, but there is some
doubt as to the former and also as regards _Moina_ (a Daphnid), in
which Grobben found a body, which he considered to be a polar body, on
the upper pole of an egg which was just entering upon segmentation. In
insects polar bodies have not been described up to the present
time[146], and only in a few cases in Vertebrata, as in _Petromyzon_ by
Kupffer and Benecke.

It must be left to the future to decide whether the expulsion of polar
bodies occurs in those large groups of animals in which they have not
been hitherto discovered. The fact, however, that they have not been so
discovered cannot be urged as an objection to my theory, for we do not
know _a priori_ whether the removal of the ovogenetic nucleoplasm has
not been effected in the course of phylogeny in some other and less
conspicuous manner. The cell-body of the polar globules is so minute in
many eggs that it was a long time before the cellular nature of these
structures was recognized[147]; and it is possible that their minute
size may point to the fact that a phyletic process of reduction has
taken place, to the end that the egg may be deprived of as little
material as possible. It is at all events proved that in all Metazoan
groups the nucleus undergoes changes during the maturation of the egg,
which are entirely similar to those which lead to the formation of
polar bodies in those eggs which possess them. In the former instances
it is possible that nature has taken a shortened route to gain the same
end.

It would be an important objection if it could be shown that no process
corresponding to the expulsion of polar bodies takes place in the male
germ-cells, for it is obvious that here also we should, according to my
theory, expect such a process to occur. The great majority of
sperm-cells differ so widely in character from the ordinary indifferent
(i. e. undifferentiated) cells, that they are evidently histologically
differentiated in a very high degree, and hence the sperm-cells, like
the yolk-forming germ-cells, must possess a specific nuclear substance.
The majority of sperm-cells therefore resemble the somatic cells in
that they have a specific histological structure, but their
characteristic form has nothing to do with their fertilizing power,
viz. with their power of being the bearers of germ-plasm. Important as
this structure is, in order to render it possible that the egg-cell may
be approached and penetrated, it has nothing to do with the property of
the sperm-cell to transmit the qualities of the species and of the
individual to the following generation. The nuclear substance which
causes such a cell to assume the appearance of a thread, or a stellate
form (in Crustacea), or a boomerang form (present in certain Daphnids),
or a conical bullet shape (Nematodes), cannot possibly be the same
nuclear substance as that which, after conjugation with the egg-cell,
contains in its molecular structure the tendency to build up a new
Metazoon of the same kind as that by which it was produced. We must,
therefore, conclude that the sperm-cell also contains two kinds of
nucleoplasm, namely, germ-plasm and spermogenetic nucleoplasm.

It is true that we cannot say _a priori_ whether the influence
exercised on the sperm-cell by the spermogenetic nucleoplasm might not
be eliminated by some means other than its removal from the cell. It is
conceivable, for instance, that this substance may be expelled from the
nucleus, but may remain in the cell-body, where it is in some way
rendered powerless. We do not yet really know anything of the essential
conditions of nuclear division, and it is quite impossible to bring
forward any facts in support of my previous suggestion. The germ-plasm
is supposed to be present in the nucleus of the growing egg-cell in
smaller quantity than the ovogenetic nucleoplasm, and the germ-plasm
gradually increases in quantity: thus when the egg has attained its
maximum size, the opposition between the two different kinds of
nucleoplasm becomes so marked, in consequence of the alteration in
their quantitative relations, that their separation, viz. nuclear
division, results. But although we are not able to distinguish, by any
visible characteristics, the different kinds of nucleoplasm which may
be united in one nuclear thread, the assumption that the influence of
each kind bears a direct proportion to its quantity is the most obvious
and natural one. The tendency of the germ-plasm contained in the
nucleus cannot make itself felt so long as an excess of ovogenetic
nucleoplasm is also present. We may imagine that the effects of the two
different kinds of nucleoplasm are combined to produce a resultant
effect; but when the two influences exerted upon the cell are nearly
opposed, only the stronger can make itself felt, and in such a case the
latter must exceed the former in quantity, because part of it is as it
were neutralized by the other nucleoplasm working in an opposite
direction. This metaphorical representation may give us a clue to
explain the fact that the ovogenetic nucleoplasm comes to exceed the
germ-plasm in quantity. For obviously these two kinds of nucleoplasm
exert opposite tendencies in at least one respect. The germ-plasm tends
to effect the division of the cell into the two first segmentation
spheres; the ovogenetic nucleoplasm, on the other hand, possesses a
tendency towards the growth of the cell-body without division. Hence
the germ-plasm cannot make itself felt, and is not able to expel the
ovogenetic nucleoplasm until it has reached such a relative size as
enables it to successfully oppose the latter.

Applying these ideas to the sperm-cells we must see whether the
expulsion of part of the nuclear substance, viz. of the spermogenetic
nucleoplasm, corresponding to the ovogenetic nucleoplasm, takes place
in them also.

As far as we can judge from thoroughly substantiated observations such
phenomena are indeed found in many cases, although they appear to be
different from those occurring in the egg-cell, and cannot receive
quite so certain an interpretation.

The attempt to prove that a process similar to the expulsion of polar
bodies takes place in the formation of sperm-cells has already been
made by those observers who regard such expulsion as the removal of the
male element from the egg, thus leading to sexual differentiation; for
such a theory also requires the removal of part of the nuclear
substance from the maturing sperm-cell. Thus, according to E. van
Beneden and Ch. Julin, the cells which, in _Ascaris_, produce the
spermatogonia (mother-cells of the sperm-cells), expel certain elements
from their nuclear plate, a phenomenon which has not been hitherto
observed in any other animal, and even in this instance has only been
inferred and not directly observed. Moreover the sperm-cells have not
attained their specific form (conical bullet-shaped) at the time when
this expulsion takes place from the spermatogonia, and we should expect
that the spermogenetic nucleoplasm would not be removed until it has
completed its work, viz. not until the specific shape of the sperm-cell
has been attained. We might rather suppose that phenomena explicable in
this way are to be witnessed in those sperm-blastophores (mother-cells
of sperm-cells) which, as has been known for a long time, are not
employed in the formation of the nuclei of sperm-cells, but for the
greater part remain at the base of the latter and perish after their
maturation and separation. In this case an influence might be exerted
by these nuclei upon the specific form of the sperm-cells, for the
former arise and develope in the form of bundles of spermatozoa in the
interior of the mother-cell.

It has been already shown in many groups of animals that parts of the
sperm-mother-cells[148] perish, without developing into sperm-cells, as
in Selachians, in the frog, in many worms and snails, and also in
mammals (Blomfield). But the attention of observers has been directed
to that part of the cell-body which is not used in the formation of
sperm-cells, rather than to the nucleus; and the proof that part of the
nucleus also perishes is still wanting in many of these cases. Fresh
investigation must decide whether the nucleus of the sperm-mother-cell
perishes as a general rule, and whether part of the nucleus is rendered
powerless in some other way, where such mother-cells do not exist.
Perhaps the paranucleus (Nebenkern) of the sperm-cell, first described
by La Valette St. George, and afterwards found in many animals of very
different groups, is the analogue of the polar body. It is true that
this so-called paranucleus is now considered as a condensed part of the
cell-body, but we must remember that it has been hitherto a question
whether the head of the spermatozoon is formed from the nucleus of the
cell or from the paranucleus; and that the observers who held the
former view were in consequence obliged to regard the paranucleus as a
product of the cell-body. But according to the most recent
investigations of Fol[149], Roule[150], Balbiani[151], and Will[152],
upon the formation of the follicular epithelium in the ovary of
different groups, it is not improbable that parts of the nucleus may
become detached without passing through the process of karyokinesis.
Thus it is very possible that the paranucleus may be a product of the
main nucleus and not a condensed part of the cell-body. This view is
supported by its behaviour with staining reagents, while the other
view, that it arises from the cell-substance, is not based upon direct
observation. Consequently future investigation must decide whether the
paranucleus is to be considered as the spermogenetic nucleoplasm
expelled from the nucleus. But even if this question is answered in the
affirmative, we should still have to explain why this nuclear
substance, remaining in the cell-body, does not continue to exercise
any control over the latter.

Strasburger has recently enumerated a large number of cases from
different groups of plants, in which the maturation of both male and
female germ-cells is accompanied by phenomena similar to the expulsion
of polar bodies. In this respect the phenomena occurring in the
pollen-grains of Phanerogams bear an astonishing resemblance to the
maturation of the animal egg. For instance, in the larch, the
sperm-mother-cell divides three times in succession, the products of
division being very unequal on each occasion; and exactly as in the
case of polar bodies, the three small so-called vegetative cells shrink
rapidly after separation, and have no further physiological value.
According to Strasburger, the so-called ‘ventral canal-cell,’ which, in
mosses, ferns, and Conifers, separates from the female germ-cell,
reminds us, in every way, of the polar bodies of animal eggs.
Furthermore, the spermatozoids in the mosses and vascular cryptogams
throw off a small vesicle before performing their functions[153]. On
the other hand the equivalents of ‘polar bodies’ (the ‘ventral
canal-cells’) are said to be absent in the Cycads, although these are
so nearly allied to Conifers. Furthermore, ‘no phenomenon occurs in the
oospheres (ova) of Angiosperms which can be compared to the formation
of polar bodies.’ Strasburger therefore concludes that the separation
of certain parts from the germ-cells is not in all cases necessary for
maturation, and that such phenomena are not fundamental, like those of
fertilization, which must always take place along the same
morphological lines. He further concludes that the former phenomena are
only necessary in the case of the germ-cells of certain organisms, in
order to bring the nuclei destined for the sexual act into the
physiological condition necessary for its due performance.

I am unwilling to abandon the idea that the expulsion of the
histogenetic parts of the nuclear substance, during the maturation of
germ-cells, is also a general phenomenon in plants; for the process
appears to be fundamental, while the argument that it has not been
proved to occur universally is only of doubtful value. The embryo-sac
of Angiosperms is such a complex structure that it seems to me to be
possible (as it does to Strasburger) that ‘processes which precede the
formation of the egg-cell have borne relation to the sexual
differentiation of the nucleus of the egg.’ Besides, it is possible
that the vegetable egg-cell may, in certain cases, possess so simple a
structure and so small a degree of histological specialization, that it
would not be necessary for it to contain any specific histogenetic
nucleoplasm: thus it would consist entirely of germ-plasm from the
first. In such cases, of course, its maturation would not be
accompanied by the expulsion of somatic nucleoplasm.

I have hitherto abstained from discussing the question as to whether
the process of the formation of polar bodies may require an
interpretation which is entirely different from that which I have given
it, whether it may receive a purely morphological interpretation.

In former times it could only be regarded as of purely phyletic
significance: it could only be looked upon as the last remnant of a
process which formerly possessed some meaning, but which is now devoid
of any physiological importance. We are indeed compelled to admit that
a process does occur in connexion with the true polar bodies of animal
eggs, which we cannot explain on physiological grounds; I mean the
division of the polar bodies after they have been expelled from the
egg. In many animals the two polar bodies divide again after their
expulsion, so as to form four bodies, which distinctly possess the
structure of cells, as Trinchese observed in the case of gastropods.
But, in the first place, this second division does not always take
place, and, secondly, it is very improbable that a process which occurs
during the first stage of ontogeny, or more properly speaking, before
the commencement of ontogeny, and which is, therefore, a remnant of
some excessively ancient phyletic stage, would have been retained up to
the present day unless it possessed some very important physiological
significance. We may safely maintain that it would have disappeared
long ago if it had been without any physiological importance. Relying
on our knowledge of the slow and gradual, although certain,
disappearance, in the course of phylogeny, of organs which have lost
their functions, and of processes which have become meaningless, we are
compelled to regard the process of the formation of polar bodies as of
high physiological importance. But this view does not exclude the
possibility that the process possessed a morphological meaning also,
and I believe that we are quite justified in attempting (as
Bütschli[154] has recently done) to discover what this morphological
meaning may have been.

Should it be finally proved that the expulsion of polar bodies is
nothing more than the removal of histogenetic nucleoplasm from the
germ-cell, the opinion (which is so intimately connected with the
theory of the continuity of the germ-plasm) that a re-transformation of
specialised idioplasm into germ-plasm cannot occur, would be still
further confirmed; for we do not find that any part of an organism is
thrown away simply because it is useless: organs that have lost their
functions are re-absorbed, and their material is thus employed to
assist in building up the organism.

                 III. On the Nature of Parthenogenesis.

It is well known that the formation of polar bodies has been repeatedly
connected with the sexuality of germ-cells, and that it has been
employed to explain the phenomena of parthenogenesis. I may now,
perhaps, be allowed to develope the views as to the nature of
parthenogenesis at which I have arrived under the influence of my
explanation of polar bodies.

The theory of parthenogenesis adopted by Minot and Balfour is
distinguished by its simplicity and clearness, among all other
interpretations which had been hitherto offered. Indeed, their
explanation follows naturally and almost as a matter of course, if the
assumption made by these observers be correct, that the polar body is
the male part of the hermaphrodite egg-cell. An egg which has lost its
male part cannot develope into an embryo until it has received a new
male part in fertilization. On the other hand, an egg which does not
expel its male part may develope without fertilization, and thus we are
led to the obvious conclusion that parthenogenesis is based upon the
non-expulsion of polar bodies. Balfour distinctly states ‘that the
function of forming polar cells has been acquired by the ovum for the
express purpose of preventing parthenogenesis[155].’

It is obvious that I cannot share this opinion, for I regard the
expulsion of polar bodies as merely the removal of the ovogenetic
nucleoplasm, on which depended the development of the specific
histological structure of the egg-cell. I must assume that the
phenomena of maturation in the parthenogenetic egg and in the sexual
egg are precisely identical, and that in both, the ovogenetic
nucleoplasm must in some way be removed before embryonic development
can begin.

Unfortunately the actual proof of this assumption is not so complete as
might be desired. In the first place, we are as yet uncertain whether
polar bodies are or are not expelled by parthenogenetic eggs[156]; for
in no single instance has such expulsion been established beyond doubt.
It is true that this deficiency does not afford any support to the
explanation of Minot and Balfour, for in all cases in which polar
bodies have not been found in parthenogenetic eggs, these structures
are also absent from the eggs which require fertilization in the same
species. But although the expulsion of polar bodies in parthenogenesis
has not yet been proved to occur, we must assume it to be nearly
certain that the phenomena of maturation, whether connected or
unconnected with the expulsion of polar bodies, are the same in the
eggs which develope parthenogenetically and in those which are capable
of fertilization, in one and the same species. This conclusion depends,
above all, upon the phenomena of reproduction in bees, in which, as a
matter of fact, the same egg may be fertilized or may develope
parthenogenetically, as I shall have occasion to describe in greater
detail at a later period.

Hence when we see that the eggs of many animals are capable of
developing without fertilization, while in other animals such
development is impossible, the difference between the two kinds of eggs
must rest upon something more than the mode of transformation of the
nucleus of the germ-cell into the first segmentation nucleus. There
are, indeed, facts which distinctly point to the conclusion that the
difference is based upon quantitative and not qualitative relations. A
large number of insects are exceptionally reproduced by the
parthenogenetic method, e. g. in Lepidoptera. Such development does not
take place in all the eggs laid by an unfertilized female, but only in
part, and generally a small fraction of the whole, while the rest die.
But among the latter there are some which enter upon embryonic
development without being able to complete it, and the stage at which
development may cease also varies. It is also known that the eggs of
higher animals may pass through the first stages of segmentation
without having been fertilized. This was shown to be the case in the
egg of the frog by Leuckart[157], in that of the fowl by
Oellacher[158], and even in the egg of mammals by Hensen[159].

Hence in such cases it is not the impulse to development, but the power
to complete it, which is absent. We know that force is always bound up
with matter, and it seems to me that such instances are best explained
by the supposition that too small an amount of that form of matter is
present, which, by its controlling agency, effects the building-up of
the embryo by the transformation of mere nutritive material. This
substance is the germ-plasm of the segmentation nucleus, and I have
assumed above that it is altered in the course of ontogeny by changes
which arise from within, so that when sufficient nourishment is
afforded by the cell-body, each succeeding stage necessarily results
from the preceding one. I believe that changes arise in the
constitution of the nucleoplasm at each cell-division which takes place
during the building-up of the embryo, changes which either correspond
or differ in the two halves of each nucleus. If, for the present, we
neglect the minute amount of unchanged germ-plasm which is reserved for
the formation of the germ-cells, it is clear that a great many
different stages in the development of somatic nucleoplasm are thus
formed, which may be denominated as stages 1, 2, 3, 4, &c., up to _n_.
In each of these stages the cells differ more as development proceeds,
and as the number by which the stage is denominated becomes higher.
Thus, for instance, the two first segmentation spheres would represent
the first stage of somatic nucleoplasm, a stage which may be considered
as but slightly different in its molecular structure from the
nucleoplasm of the segmentation nucleus; the four first segmentation
spheres would represent the second stage; the succeeding eight spheres
the third, and so on. It is clear that at each successive stage the
molecular structure of the nucleoplasm must be further removed from
that of the germ-plasm, and that, at the same time, the cells of each
successive stage must also diverge more widely among themselves in the
molecular structure of their nucleoplasm. Early in development each
cell must possess its own peculiar nucleoplasm, for the further course
of development is peculiar to each cell. It is only in the later stages
that equivalent or nearly equivalent cells are formed in large numbers,
cells in which we must also suppose the existence of equivalent
nucleoplasm.

If we may assume that a certain amount of germ-plasm must be contained
in the segmentation nucleus in order to complete the whole process of
the ontogenetic differentiation of this substance; if we may further
assume that the quantity of germ-plasm in the segmentation nucleus
varies in different cases; then we should be able to understand why one
egg can only develope after fertilization, while another can begin its
development without fertilization, but cannot finish it, and why a
third is even able to complete its development. We should also
understand why one egg only passes through the first stages of
segmentation and is then arrested, while another reaches a few more
stages in advance, and a third developes so far that the embryo is
nearly completely formed. These differences would depend upon the
extent to which the germ-plasm, originally present in the egg, was
sufficient for the development of the latter; development will be
arrested as soon as the nucleoplasm is no longer capable of producing
the succeeding stage, and is thus unable to enter upon the following
nuclear division.

From a general point of view such a theory would explain many
difficulties, and it would render possible an explanation of the
phyletic origin of parthenogenesis, and an adequate understanding of
the strange and often apparently abrupt and arbitrary manner of its
occurrence. In my works on _Daphnidae_ I have already laid especial
stress upon the proposition that parthenogenesis in insects and
Crustacea certainly cannot be an ancestral condition which has been
transmitted by heredity, but that it has been derived from a sexual
condition. In what other way can we explain the fact that
parthenogenesis is present in certain species or genera, but absent in
others closely allied to them; or the fact that males are entirely
wanting in species of which the females possess a complete apparatus
for fertilization? I will not repeat all the arguments with which I
attempted to support this conclusion[160]. Such a conclusion may be
almost certainly accepted for the _Daphnidae_, because parthenogenesis
does not occur in their still living ancestors, the Phyllopods, and
especially the _Estheridae_. In _Daphnidae_ the cause and object of the
phyletic development of parthenogenesis may be traced more clearly than
in any other group of animals. In _Daphnidae_ we can accept the
conclusion with greater certainty than in all other groups, except
perhaps the _Aphidae_, that parthenogenesis is extremely advantageous
to species in certain conditions of life; and that it has only been
adopted when, and as far as, it has been beneficial; and further, that
at least in this group parthenogenesis became possible, and was
adopted, in each species as soon as it became useful. Such a result can
be easily understood if it is only the presence of more or less
germ-plasm which decides whether an egg is, or is not, capable of
development without fertilization.

If we now examine the foundations of this hypothesis we shall find that
we may at once accept one of its assumptions, viz. that fluctuations
occur in the quantity of germ-plasm in the segmentation nucleus; for
there can never be absolute equality in any single part of different
individuals. As soon therefore as these fluctuations become so great
that parthenogenesis is produced, it may become, by the operation of
natural selection, the chief mode of reproduction of the species or of
certain generations of the species. In order to place this theory upon
a firm basis, we have simply to decide whether the quantity of
germ-plasm contained in the segmentation nucleus is the factor which
determines development; although for the present it will be sufficient
if we can render this view to some extent probable, and show that it is
not in contradiction with established facts.

At first sight this hypothesis seems to encounter serious difficulties.
It will be objected that neither the beginning nor the end of embryonic
development can possibly depend upon the quantity of nucleoplasm in the
segmentation nucleus, since the amount may be continually increased by
growth; for it is well known that during embryonic development the
nuclear substance increases with astonishing rapidity. By an
approximate calculation I found[161] that, in the egg of a _Cynips_,
the quantity of nuclear substance present at the time when the
blastoderm was about to be formed, and when there were twenty-six
nuclei, was even then seven times as great as the quantity which had
been contained in the segmentation nucleus. How then can we imagine
that embryonic development would ever be arrested from want of nuclear
substance, and if such deficiency really acted as an arresting force,
how then could development begin at all? We might suppose that when
germ-plasm is present in sufficient quantity to start segmentation, it
must also be sufficient to complete the development; for it grows
continuously, and must presumably always possess a power equal to that
which it possessed at the beginning, and which was just sufficient to
start the process of segmentation. If at each ontogenetic stage, the
quantity of nucleoplasm is just sufficient to produce the following
stage, we might well imagine that the whole ontogeny would necessarily
be completed.

The flaw in this argument lies in the erroneous assumption that the
growth of nuclear substance is, when the quality of the nucleus and the
conditions of nutrition are equal, unlimited and uncontrolled. The
intensity of growth must depend upon the quantity of nuclear substance
with which growth and the phenomena of segmentation commenced. There
must be an optimum quantity of nucleoplasm with which the growth of the
nucleus proceeds most favourably and rapidly, and this optimum will be
represented in the normal size of the segmentation nucleus. Such a size
is just sufficient to produce, in a certain time and under certain
external conditions, the nuclear substance necessary for the
construction of the embryo, and to start the long series of
cell-divisions. When the segmentation nucleus is smaller, but large
enough to enter upon segmentation, the nuclei of the two first
embryonic cells will fall rather more below the normal size, because
the growth of the segmentation nucleus during and after division will
be less rapid on account of its unusually small size. The succeeding
generations of nuclei will depart more and more from the normal size in
each respective stage, because they do not pass into a resting-stage
during embryonic development, but divide again immediately after their
formation. Hence nuclear growth would become less vigorous as the
nuclei fell more and more below the optimum size, and at last a moment
would arrive when they would be unable to divide, or would be at least
unable to control the cell-body in such a manner as to lead to its
division.

The first event of importance for embryonic development is the
maturation of the egg, i. e. the transformation of the nucleus of the
germ-cell into a nuclear spindle and the removal of the ovogenetic
nucleoplasm by the separation of polar bodies, or by some analogous
process. There must be some cause for this separation, and I have
already tried to show that it may lie in the quantitative relations
which obtain between the two kinds of nucleoplasm contained in the
nucleus of the egg. I have suggested that the germ-plasm, at first
small in quantity, undergoes a gradual increase, so that it can finally
oppose the ovogenetic nucleoplasm. I will not further elaborate this
suggestion, for the ascertained facts are insufficient for the purpose.
But the appearances witnessed in nuclear division indicate that there
are opposing forces, and that such a contest is the motive cause of
division; and Roux[162] may be right in referring the opposition to
electrical forces. However this may be, it is perfectly certain that
the development of this opposition is based upon internal conditions
arising during growth in the nucleus itself. The quantity of nuclear
thread cannot by itself determine whether the nucleus can or cannot
enter upon division; if so, it would be impossible for two divisions to
follow each other in rapid succession, as is actually the case in the
separation of the two polar bodies, and also in their subsequent
division. In addition to the effects of quantity, the internal
conditions of the nucleus must also play an important part in these
phenomena. Quantity alone does not necessarily produce nuclear
division, or the nucleus of the egg would divide long before maturation
is complete, for it contains much more nucleoplasm than the female
pronucleus, which remains in the egg after the expulsion of the polar
bodies, and which is in most cases incapable of further division. But
the fact that segmentation begins immediately after the conjugation of
male and female pronuclei, also shows that quantity is an essential
requisite. The effect of fertilization has been represented as
analogous to that of the spark which kindles the gunpowder. In the
latter case an explosion ensues, in the former segmentation begins.
Even now, many authorities are inclined to refer the polar repulsion
manifested in the nuclear division which immediately follows
fertilization, to the antagonism between male and female elements. But,
according to the important discoveries of Flemming and van Beneden, the
polar repulsion in each nuclear division is not based on the antagonism
between male and female loops, but depends upon the antagonism and
mutual repulsion between the two halves of the same loop. The loops of
the father and those of the mother remain together and divide together
throughout the whole ontogeny.

What can be the explanation of the fact that nuclear division follows
immediately after fertilization, but that without fertilization it does
not occur in most cases? There is only one possible explanation, viz.
the fact that the quantity of the nucleus has been suddenly doubled, as
the result of conjugation. The difference between the male and female
pronuclei cannot serve as an explanation, even though the nature of
this difference is entirely unknown, because polar repulsion is not
developed between the male and female halves of the nucleus, but within
each male and each female half. We are thus forced to conclude that
increase in the quantity of the nucleus affords an impulse for
division, the disposition towards it being already present. It seems to
me that this view does not encounter any theoretical difficulties, and
that it is an entirely feasible hypothesis to suppose that, besides the
internal conditions of the nucleus, its quantitative relation to the
cell-body must be taken into especial account. It is imaginable, or
perhaps even probable, that the nucleus enters upon division as soon as
its idioplasm has attained a certain strength, quite apart from the
supposition that certain internal conditions are necessary for this
end. As above stated, such conditions may be present, but division may
not occur because the right quantitative relation between nucleus and
cell-body, or between the different kinds of nuclear idioplasm, has not
been established. I imagine that such a quantitative deficiency exists
in an egg, which, after the expulsion of the ovogenetic nucleoplasm in
the polar bodies, requires fertilization in order to begin
segmentation. The fact that the polar bodies were expelled proves that
the quantity of the nucleus was sufficient to cause division, while
afterwards it was no longer sufficient to produce such a result.

This suggestion will be made still clearer by an example. In _Ascaris
megalocephala_ the nuclear substance of the female pronucleus forms two
loops, and the male pronucleus does the same; hence the segmentation
nucleus contains four loops, and this is also the case with the first
segmentation spheres. If we suppose that in embryonic development, the
first nuclear division requires such an amount of nuclear substance as
is necessary for the formation of four loops,—it follows that an egg,
which can only form two or three loops from its nuclear reticulum,
would not be able to develope parthenogenetically, and that not even
the first division would take place. If we further suppose that, while
four loops are sufficient to start nuclear division, these loops must
be of a certain size and quantity in order to complete the whole
ontogeny (in a certain species), it follows that eggs possessing a
reticulum which contains barely enough nuclear substance to divide into
four segments, would be able to produce the first division and perhaps
also the second and third, or some later division, but that at a
certain point during ontogeny, the nuclear substance would become
insufficient, and development would be arrested. This will occur in
eggs which enter upon development without fertilization, but are
arrested before its completion. One might compare this retardation
leading to the final arrest of development, to a railway train which is
intended to meet a number of other trains at various junctions, and
which can only travel slowly because of some defect in the engine. It
will be a little behind time at the first junction, but it may just
catch the train, and it may also catch the second or even the third;
but it will be later at each successive junction, and will finally
arrive too late for a certain train; and after that it will miss all
the trains at the remaining junctions. The nuclear substance grows
continuously during development, but the rate at which it increases
depends upon the nutritive conditions together with its initial
quantity. The nutritive changes during the development of an egg depend
upon the quantity of the cell-body which was present at the outset, and
which cannot be increased. If the quantity of the nuclear substance is
rather too small at the beginning, it will become more and more
insufficient in succeeding stages, as its growth becomes less vigorous,
and differs more from the standard it would have reached if the
original quantity had been normal. Consequently it will gradually fall
more and more short of the normal quantity, like the train which
arrives later and later at each successive junction, because its
engine, although with the full pressure of steam, is unable to attain
the normal speed.

It will be objected that four loops cannot be necessary for nuclear
division in _Ascaris_, since such division takes place in the formation
of the polar bodies, resulting in the appearance of the female
pronucleus with only two loops. But this fact only shows that the
quantity of nuclear substance necessary for the formation of four loops
is not necessary for all nuclear divisions; it does not disprove the
assumption that such a quantity is required for the division of the
segmentation nucleus. In addition to these considerations we must not
leave the substance of the cell-body altogether out of account, for,
although it is not the bearer of the tendencies of heredity, it must be
necessary for every change undergone by the nucleus, and it surely also
possesses the power of influencing changes to a large extent. There
must be some reason for the fact that in all animal eggs with which we
are acquainted, the nucleus moves to the surface of the egg at the time
of maturation, and there passes through its well-known transformation.
It is obvious that it is there subjected to different influences from
those which would have acted upon it in the centre of the cell-body,
and it is clear that such an unequal cell-division as takes place in
the separation of the polar bodies could not occur if the nucleus
remained in the centre of the egg.

This explanation of the necessity for fertilization does not exclude
the possibility, that, under certain circumstances, the substance of
the egg-nucleus may be larger, so that it is capable of forming four
loops. Eggs which thus possess sufficient nucleoplasm, viz. germ-plasm,
for the formation of the requisite four loops of normal size, (namely,
of the size which would have been produced by fertilization), can and
must develope by the parthenogenetic method.

Of course the assumption that four loops must be formed has only been
made for the sake of illustration. We do not yet know whether there are
always exactly four loops in the segmentation nucleus[163]. I may add
that, although the details by which these considerations are
illustrated are based on arbitrary assumptions, the fundamental view
that the development of the egg depends, _ceteris paribus_, upon the
quantity of nuclear substance, is certainly right, and follows as a
necessary conclusion from the ascertained facts. It is not unlikely
that such a view may receive direct proof in the results of future
investigations. Such proof might for instance be forthcoming if we were
to ascertain, in the same species, the number of loops present in the
segmentation nucleus of fertilization, as compared with those present
in the segmentation nucleus of parthenogenesis.

The reproductive process in bees will perhaps be used as an argument
against my theory. In these insects, the same egg will develope into a
female or male individual, according as fertilization has or has not
taken place, respectively. Hence, one and the same egg is capable of
fertilization, and also of parthenogenetic development, if it does not
receive a spermatozoon. It is in the power of the queen-bee to produce
male or female individuals: by an act of will she decides whether the
egg she is laying is to be fertilized or unfertilized. She ‘knows
beforehand’[164] whether an egg will develope into a male or a female
animal, and deposits the latter kind in the cells of queens and
workers, the former in the cells of drones. It has been shown by the
discoveries of Leuckart and von Siebold that all the eggs are capable
of developing into male individuals, and that they are only transformed
into ‘female eggs’ by fertilization. This fact seems to be incompatible
with my theory as to the cause of parthenogenesis, for if the same egg,
possessing exactly the same contents, and above all the same
segmentation nucleus, may develope sexually or parthenogenetically, it
appears that the power of parthenogenetic development must depend on
some factor other than the quantity of germ-plasm.

Although this appears to be the case, I believe that my theory
encounters no real difficulty. I have no doubt whatever, that the same
egg may develope with or without fertilization. From a careful study of
the numerous excellent investigations upon this point which have been
conducted in a particularly striking manner by Bessels[165] (in
addition to the observers quoted above), I have come to the conclusion
that the fact is absolutely certain. It must be candidly admitted that
the _same_ egg will develope into a drone when not fertilized, or into
a worker or queen when fertilized. One of Bessels’ experiments is
sufficient to prove this assertion. He cut off the wings of a young
queen and thus rendered her incapable of taking ‘the nuptial flight.’
He then observed that all the eggs which she laid developed into male
individuals. This experiment was made in order to prove that drones are
produced by unfertilized eggs; but it also proves that the assertion
mentioned above is correct, for the eggs which ripen first and are
therefore first laid, would have been fertilized had the queen been
impregnated. The supposition that, at certain times, the queen produces
eggs requiring fertilization, while at other times her eggs develope
parthenogenetically, is quite excluded by this experiment; for it
follows from it, that the eggs must all be of precisely the same kind,
and that there is no difference between the eggs which require
fertilization and those which do not.

But does it therefore follow that the quantity of germ-plasm in the
segmentation nucleus is not the factor which determines the beginning
of embryonic development? I believe not. It can be very well imagined
that the nucleus of the egg, having expelled the ovogenetic
nucleoplasm, may be increased to the size requisite for the
segmentation nucleus in one of two ways: either by conjugation with a
sperm-nucleus, or by simply growing to double its size. There is
nothing improbable in this latter assumption, and one is even inclined
to inquire why such growth does not take place in all unfertilized
eggs. The true answer to this question must be that nature generally
pursues the sexual method of reproduction, and that the only way in
which the general occurrence of parthenogenesis could be prevented, was
by the production of eggs which remained sterile unless they were
fertilized. This was effected by a loss of the capability of growth on
the part of the egg-nucleus after it had expelled the ovogenetic
nucleoplasm.

The case of the bee proves in a very striking manner that the
difference between eggs which require fertilization, and those which do
not, is not produced until after the maturation of the egg, and the
removal of the ovogenetic nucleoplasm. The increase in the quantity of
the germ-plasm cannot have taken place at any earlier period, or else
the nucleus of the egg would always start embryonic development by
itself, and the egg would probably be incapable of fertilization. For
the relation between egg-nucleus and sperm-nucleus is obviously based
upon the fact that each of them is insufficient by itself, and requires
completion. If such completion had taken place at an early stage the
egg-nucleus would either cease to exercise any attractive force upon
the sperm-nucleus, or else conjugation would be effected, as in Fol’s
interesting experiments upon fertilization by many spermatozoa; and, as
in these experiments, malformation of the embryo would result. In
_Daphnidae_ I believe I have shown[166] that the summer-eggs are not
only developed parthenogenetically, but also that they are never
fertilized; and the explanation of this incapacity for fertilization
may perhaps be found in the fact that their segmentation nucleus is
already formed.

We may therefore conclude that, in bees, the nucleus of the egg, formed
during maturation, may either conjugate with the sperm-nucleus, or else
if no spermatozoon reaches the egg may, under the stimulus of internal
causes, grow to double its size, thus attaining the dimensions of the
segmentation nucleus. For our present purpose we may leave out of
consideration the fact that in the latter case the individual produced
is a male, and in the former case a female.

It is clear that such an increase in the germ-plasm must depend, to a
certain extent, upon the nutrition of the nucleus, and thus indirectly
upon the body of the egg-cell; but the increase must chiefly depend
upon internal nuclear conditions, viz. upon the capability of growth.
We must further assume that the latter condition plays the chief part
in the process, for everywhere in the organic world the limit of growth
depends upon the internal conditions of the growing body, and can only
be altered to a small extent by differences of nutrition. The phyletic
acquisition of the capability of parthenogenetic development must
therefore depend upon an alteration in the capability of growth
possessed by the nucleus of the egg.

This theory of parthenogenesis most nearly approaches Strasburger’s
views upon the subject, for he also explains the non-occurrence of
parthenogenetic development by the insufficient quantity of nucleoplasm
remaining in the egg after the expulsion of polar bodies. The former
theory differs however in that the occurrence of parthenogenesis is
supposed to be only due to an increase of this nucleoplasm to the
normal size of the segmentation nucleus. Strasburger assumes that
‘specially favourable conditions of nutrition counteract the deficiency
of nuclear idioplasm,’ while it seems to me that nutrition must be
considered as only of secondary importance. Thus in bees, as above
stated, the same egg may develope parthenogenetically or after
fertilization, the nucleus being subject to the same conditions of
nutrition in both cases. Strasburger[167] considers that
parthenogenesis may be interpreted by one of three possible
explanations. First, he suggests that especially favourable nutrition
may lead to the completion of the nuclear idioplasm. But if this
assumption be made, we must ask why a part of the idioplasm should be
previously expelled, when immediately afterwards the presence of an
equal amount becomes necessary. Such a view can only be explained by
the above-made assumption that the expelled nucleoplasm has a different
constitution from that possessed by the nucleoplasm which is afterwards
formed. It is true that we do not yet certainly know whether a polar
body is expelled in eggs in which parthenogenesis occurs, but we do
know that the egg of the bee passes through the same stages of
maturation whether it is to be fertilized or not. I can hardly accept
Strasburger’s second suggestion, ‘that under some favourable conditions
of nutrition half [or perhaps better, a quarter] of the idioplasm of
the egg-nucleus is sufficient to start the processes of development in
the cyto-idioplasm.’ Finally, his third suggestion, ‘that the
cyto-idioplasm, nourished by its surroundings and thus increased in
quantity, compels the nucleus of the egg to enter upon division,’
presupposes that the cell-body gives the impulse for nuclear division,
a supposition which up to the present time remains at least unproved.
The ascertained facts appear to me to indicate rather that the
cell-body serves only as a medium for the nutrition of the nucleus, and
Fol’s recently mentioned observations, which have been especially
quoted by Strasburger in support of his theories, seem to me to rather
confirm my conclusions. If supernumerary sperm-nuclei penetrate into
the egg, they may, under the nutritive influence of the cell-body,
become centres of attraction, and may take the first step towards
nuclear and cell-division by forming amphiasters. Such nuclei cannot
control the whole cell-body and force it to divide, but each one of
them, having grown to a certain size at the expense of the cell-body,
makes its influence felt over a certain area. Strasburger is quite
right in considering this process as a ‘partial parthenogenesis.’ Such
partial parthenogenesis presumably occurs in all egg-nuclei, but the
latter cannot attain to complete parthenogenesis when, as in Fol’s
supernumerary sperm-nuclei, their powers of assimilation are
insufficient to enable them to reach the requisite size. As before
stated, the cell-body does not force the nucleus to divide, but _vice
versa_. It would, moreover, be quite erroneous to suppose that
parthenogenetic eggs must contain a larger amount of nutritive material
in order to facilitate the growth of the nucleus. The parthenogenetic
eggs of certain _Daphnidae_ (_Bythotrephes_, _Polyphemus_) are very
much smaller than the winter-eggs, which require fertilization, in the
same species. It is also an error for Strasburger to conclude that ‘it
has been established with certainty that favourable conditions of
nutrition cause parthenogenetic development in _Daphnidae_, while
unfavourable conditions cause the formation of eggs requiring
fertilization.’ It is true that Carl Düsing[168], in his notable work
upon the origin of sex, has attempted, in a most ingenious manner, to
prove, from my observations and experiments on the reproduction of
_Daphnidae_, ‘that winter or summer-eggs are formed according to the
nutritive condition of the ovary.’ I do not, however, believe that he
has succeeded in this attempt, and at all events it is quite clear that
the validity of such conclusions is not fully established. I have
observed that the maturing eggs break up in the ovaries and are
absorbed in those _Daphnidae_ (_Sida_) which are starved because
sufficient food cannot be provided in captivity. Hence such animals
live, as it were, at the expense of their descendants; but it would be
quite erroneous to conclude with Düsing, from the similarity which such
disappearing egg-follicles bear to the groups of germ-cells which
normally break up in the formation of winter-eggs, that with a less
degree of starvation winter-eggs would have been formed. Düsing further
quotes my incidental remark that the formation of resting-eggs in
_Daphnia_ has been especially frequent in aquaria ‘which had been for
some time neglected, and in which it was found that a great increase in
the number of individuals had taken place.’ He is entirely wrong in
concluding that there was any want of food in these neglected aquaria;
and if I had foreseen that such conclusions would have been drawn, I
might have easily guarded against them by adding that in these very
aquaria an undisturbed growth of different algae was flourishing, so
that there could have been no deficiency, but, on the contrary, a great
abundance of nutritive material. I may add that since that time I have
conducted some experiments directly bearing upon this question, by
bringing virgin females as near to the verge of starvation as possible,
but in no case did they enter upon sexual reproduction[169].

An author must have been to some extent misled by preconceived ideas
when he is unable to see that the manner in which the two kinds of eggs
are respectively formed, directly excludes the possibility of the
origin of sexual eggs from the effects of deficient or poor nutrition.
The resting eggs, which require fertilization, are always larger, and
require for their formation far more nutritive material, than the
parthenogenetic summer-eggs. In _Moina_, for instance, forty large
food-cells are necessary for the formation of a resting egg, while a
summer-egg only requires three. And Düsing is aware of these facts, and
quotes them. How can the formation of resting eggs depend upon the
effects of poor nutrition when food is most abundant at the very time
of their formation? In all those species which inhabit lakes, sexual
reproduction occurs towards the autumn, and in such cases the resting
eggs are true winter-eggs, destined to preserve the species during the
winter. But at no time of the year is the food of the _Daphnidae_ so
abundant as in September and October, and frequently even until late in
November (in South Germany). At this period of the year, the water is
filled with flakes of animal and vegetable matter in a state of partial
decomposition, thus affording abundant food for many species. It also
swarms with a large number of species of Crustacea, Radiolaria, and
Infusoria; and thus such Daphnids as the _Polyphemidae_ are also well
provided for. Hence there is no deficiency in the supply of food. Any
one who has used a fine net in our fresh waters at this time of the
year must have been at first astonished at the enormous abundance of
the lower forms of animal life; and he must have been much more
astonished if he has been able to compare such results with the scanty
population of the same localities in spring. But it is during the
spring and summer that these very _Daphnidae_ reproduce themselves
parthenogenetically. I am far from believing that my experiments on
_Daphnidae_ are exhaustive and final, and I have stated this in my
published writings on the subject; but it seems to me that I have
established the fact that direct influences, whether of food or of
temperature, acting upon single individuals, do not determine the kind
of eggs which are to be produced; but that such a decisive influence is
to be found in the indirect conditions of life, and especially in the
average frequency of the recurrence of adverse circumstances which kill
whole colonies at once, such as the winter cold, or the drying-up of
small ponds in summer. It is unnecessary for me to controvert Düsing in
detail, as I have already taken this course in the case of Herbert
Spencer[170], who had also formed the hypothesis that diminished
nutrition causes sexual reproduction.

One of my observations seems, indeed, to support such a view, but only
when it is considered as an isolated example. I refer to the behaviour
of the genus _Moina_. Females of this genus which possess sexual eggs
in their ovaries, and which would have continued to produce such eggs
if males had been present, enter in the absence of the latter upon the
formation of parthenogenetic summer-eggs, that is, if the sexual eggs
have not all been extruded, but have been re-absorbed in the ovary. At
first sight, indeed, such a result appears to indicate that the
increase in nutrition, produced by the breaking-up of the large
winter-egg in the ovary, determines the formation of parthenogenetic
eggs. This apparent conclusion seems to be further confirmed by the
following fact. The transition from sexual to parthenogenetic
reproduction only occurs in one species of _Moina_ (_M. rectirostris_),
but in this species it occurs always and without exception, while in
the other species which I have investigated (_M. paradoxa_),
winter-eggs, when once formed, are always laid, and such females can
never produce summer-eggs. But in spite of this fact, Düsing is
mistaken when he explains the continuous formation of sexual eggs in
the latter species as due to the absence of any great increase in the
amount of nutrition, such as would have followed if the egg had broken
up in the ovary. In many other _Daphnidae_ which have come under my
notice, the females frequently enter again upon the formation of
parthenogenetic summer-eggs, after having laid fertilized resting eggs,
upon one or more occasions. This is the case, for instance, in all the
species of _Daphnia_ with which I am acquainted, and such a fact at
once proves that the abnormal increase in nutrition produced by the
absorption of winter-eggs cannot be the cause of the succeeding
parthenogenesis. It also supports the proof that a high or low
nutritive condition of the whole animal can have nothing to do with the
kind of eggs which are produced, for in the above-quoted instance, the
nutrition has remained the same throughout, or at all events has not
been increased. It is erroneous to always look for the explanation of
the mode of egg-formation in the direct action of external causes. Of
course there must be direct causes which determine that one germ shall
become a winter-egg, and another a summer-egg; but such causes do not
lie outside the animal, and have nothing to do with the nutritive
condition of the ovary: they are to be found in those conditions which
we are not at present able to analyze further, and which we must, in
the meantime, call the specific constitution of the species. In the
young males of _Daphnidae_ the testes have precisely the same
appearance as the ovaries of the young females[171], but the former
will, nevertheless, produce sperm-cells and not ova. In such cases the
sex of the young individual can always be identified by the form of the
first antenna and of the first thoracic appendage, both of which are
always clawed in the male. But who can point to the direct causes which
determine that the sexual cells shall become sperm-cells in this case,
and not egg-cells? Does the determining cause depend on the conditions
of nutrition? Or, again, in the females, can the state of nutrition
determine that the third out of a group of four germ-cells shall become
an egg-cell, and that the others shall break up to serve as its food?

It is, I think, clear that these are obvious instances of the general
conclusion that the direct causes determining the direction of
development in each case are not to be looked for in external
conditions, but in the constitution of the organs concerned.

We arrive at a like conclusion when we consider the quality of the eggs
which are produced. The constitution of one species of _Moina_ contains
the cause which determines that each individual shall produce
winter-eggs only, or summer-eggs only; while in another species the
transition from the formation of sexual eggs to the formation of
summer-eggs can take place, but only when the winter-egg remains
unfertilized. The latter case appears to me to be notably a special
adaptation, in this and other species, to the deficiency of males,
which is apt to occur. At all events, it is obvious that it is an
advantage that an unfertilized sexual egg shall not be lost to the
organism. The re-absorption of the winter-egg is an arrangement which,
without being the cause, is favourable to the production of summer-eggs.

This subject is by no means a simple one, as is proved by the behaviour
of the small group of _Daphnidae_. Thus in some species, the
winter-eggs are produced by purely sexual females, which never enter
upon parthenogenesis; in others, the sexual females may take the latter
course, but only when males are absent; in others, again, they
regularly enter upon parthenogenesis. In my work on _Daphnidae_, I have
attempted to show that their behaviour in this respect is associated
with the various external conditions under which the different species
live; and also that the ultimate occurrence of the sexual period, and
finally the whole cyclical alternation of sexual and parthenogenetic
reproduction, depend upon adaptation to certain external conditions of
life.

With the aid of my hypothesis that the egg-nucleus is composed of
ovogenetic nucleoplasm and germ-plasm, I can now attempt to give an
approximate explanation of the nature and origin of the direct causes
which determine the production, at one time of parthenogenetic
summer-eggs, and at another time of winter-eggs, requiring
fertilization. But in such an explanation I should also wish to include
a consideration of the causes which determine the formation of the
nutritive cells of the egg and of the sperm-cells to which I have
alluded above.

I believe that the direct cause which determines why the apparently
identical cells of the young testis and ovary in the _Daphnidae_
develope in such different directions, is to be found in the fact, that
their nuclei possess different histogenetic nucleoplasms, while, if we
neglect individual differences, the germ-plasm remains precisely the
same. In the sperm-cells the histogenetic nucleoplasm is spermogenetic,
in the egg-cells it is ovogenetic. This must be conceded if our
fundamental view is correct, that the specific nature of the cell-body
is determined by the nature of its nucleus.

Similarly, the germ-cells of female _Daphnidae_, which at first do not
exhibit the smallest differences, must really differ in that their
nuclei must contain different kinds of nucleoplasm, which are present
in different proportions. Germ-cells which are to produce a finely
granular, brick-red, winter yolk (_Moina rectirostris_) must possess an
ovogenetic nucleoplasm of a somewhat different molecular structure from
those germ-cells which have only to form a few large blue fat-globules,
as in the summer-eggs of the same species. It is further probable that
different proportions obtain between germ-plasm and ovogenetic
nucleoplasm, in these two kinds of germ-cells; and it would be a very
simple explanation of the otherwise obscure part played by the
food-cells, if we were to suppose that they do not contain any
germ-plasm at all, and on this account do not enter upon embryonic
development, but are arrested after growing to a certain size. Such an
explanation, however, would not by itself show why they subsequently
undergo gradual solution in the surrounding fluids. But since we know
that egg-cells also begin to undergo solution as soon as the parent
Daphnid is poorly nourished, we can hardly help also referring the
solution of the food-cells to insufficient nourishment, occurring as
soon as the egg-cell, after the attainment of a certain size, exercises
a superior power of assimilation. But hitherto we could not in any way
understand why the third out of a group of germ-cells should always
gain this superior power and become an egg-cell. If it could be shown
that its position is more highly favoured in respect of nutrition, we
could understand why it outstrips the other three in development, and
thus prevents them from further growth. But nothing of the kind can be
shown to occur with any degree of probability, as I have previously
mentioned in my works on the subject. At that time, having no better
explanation, I adopted the view in question, although only as a
provisional interpretation. It was not possible for me to seek in the
substance of those four apparently identical cells for the cause of
their different development; but now I am justified in offering the
supposition that during the division of a primitive germ-cell into two,
and afterwards into four germ-cells, an unequal division of the
nucleoplasms takes place, in that one of the four cells receives
germ-plasm as well as ovogenetic nucleoplasm, while the other three
receive the latter alone. Similarly, the fact that the second cell of
the group may occasionally become an egg is also intelligible, although
this fact remained quite inexplicable by my former interpretation. The
fact that true egg-cells, or even the whole ovary with all its
germ-cells, may break up and become absorbed when the animal has been
starved for a certain period of time, seems to me to be no objection to
our present view, any more than the fact that an Infusorian may die
from starvation would be an objection to the supposition of the
immortality of unicellular organisms. The growth of an organism is not
only arrested by its constitution, but also by absolute want of food;
but it would be very foolish to explain the differences in size of the
various species of animals as results of the different conditions of
nutrition to which they were subject. Just as a sparrow, however highly
nourished, could never attain the size or form of an eagle, so a
germ-cell destined to become a summer-egg could never attain the size,
form, or colour of a winter-egg. It is by internal constitutional
causes that the course of development is determined in both these
cases; and in the latter, the cause can hardly be anything more than
the different constitution of the nucleoplasms.

All these considerations depend upon the supposition that the
egg-nucleus contains two kinds of idioplasm, viz. germ-plasm and
ovogenetic nucleoplasm. I have not hitherto brought forward any direct
evidence in favour of this assumption, but I believe that such proofs
can be obtained.

It is well known that there are certain eggs in which the polar bodies
are not expelled until after the entrance of spermatozoa. Brooks[172]
has already made use of this fact as evidence against Minot’s and
Balfour’s theory; for he quite rightly concludes that if the polar
bodies really possess the significance of male cells, we cannot
understand why such eggs are unable to develope without fertilization,
when they still possess the male half of the nucleus necessary for
development. But such eggs (e.g. that of the oyster) do not develope,
but always die if they remain unfertilized.

This argument can only be met by a new hypothesis, the construction of
which I must leave to the defenders of the above-mentioned theory. But
the observation in question seems to me to furnish at the same time a
proof of the co-existence of two different nucleoplasms in the
egg-nucleus. If the nucleoplasm of the polar bodies was also
germ-plasm, we could not understand why such eggs are unable to
develope parthenogenetically, for at least as much germ-plasm is
contained in the unfertilized egg as would have been present after
fertilization.

The only objection which can be raised against this conclusion depends
upon the supposition that the nucleoplasm of the sperm-cell is
qualitatively different from that of the egg-cell. I have already dealt
with this view, but I should wish to refer to it again rather more in
detail. Some years ago I expressed the opinion[173] that the
physiological values of the sperm-cell and of the egg-cell must be
identical; that they stand in the ratio of 1 : 1. But Valaoritis[174]
has brought forward the objection that if we consider the function of a
cell as the measure of its physiological value, it is only necessary to
point to the respective functions of ovum and spermatozoon in order to
show that their physiological values must be different. ‘The egg-cell
alone, by passing more or less completely through the phyletic stages
of the female parent, developes into a similar organism; and although
the presence of the spermatozoon is in most cases required in order to
render possible such a result, the cases of parthenogenesis prove
nevertheless that the egg can do without this stimulus.’ This objection
appeared to be fully justified as long as fertilization was looked upon
as the ‘vitalization of the germ,’ and so long as the sperm-cell was
considered as merely ‘the spark that kindles the gunpowder,’ and
further so long as the germ-substance was believed to be contained in
the cell-body. But now we can hardly give to the body of the egg-cell a
higher significance than that of the common nutritive soil of the two
nuclei which conjugate in fertilization. But these two nuclei ‘are not
different in nature,’ as Strasburger says, and as I fully believe. They
cannot differ in kind, for they both consist of germ-plasm belonging to
the same species of animal or plant; and there cannot be any deeper
contrast between them such as would correspond to the differences
between mature individuals. They cannot, from their essential nature,
exercise any special attraction upon each other, and when we see that
sperm-cell and egg-cell do nevertheless attract each other, as has been
shown in both plants and animals, such a property must have been
secondarily acquired, and has no other significance than to favour the
union of sexual cells—an arrangement which may be compared to the
vibrating flagellum of the spermatozoon or the micropyle of the egg,
but which is not fundamental, and is not based upon the molecular
structure of the germ-plasm. In lower plants, Pfeffer has proved that
certain chemical stimuli emanate from the egg and attract the
spermatozoid; and according to Strasburger, the synergidae in the upper
part of the embryo-sac of Phanerogams secrete a substance which is
capable of directing the growth of the pollen-tube towards the
egg-cell. In animals it is only known as yet that spermatozoa and ova
do attract each other, so that the former find the latter and bore
their way through its membranes. It has also been shown that the
substance of the egg-body moves towards the penetrating spermatozoon
(‘_cones d’exsudation_’ in _Asteridae_: Fol); and that it sometimes
enters upon convulsive movements (_Petromyzon_). Here therefore a
mutual stimulation and attraction must exist; and perhaps we must also
assume that there is an attraction between the two conjugating nuclei,
for we cannot readily understand how the cytoplasm alone could direct
the one to the other, as Strasburger supposes. According to
Strasburger’s hypothesis, we must suppose that part of the specific
cytoplasm of the sperm-cell continues to surround the nucleus after it
has penetrated into the body of the egg. But however this may be, the
assumed attraction between the conjugating nuclei certainly cannot
depend upon the molecular structure of their germ-plasm, which is the
same in both, but it must be due to some accessory circumstance. If it
were possible to introduce the female pronucleus of an egg into another
egg of the same species, immediately after the transformation of the
nucleus of the latter into the female pronucleus, it is very probable
that the two nuclei would conjugate just as if a fertilizing
sperm-nucleus had penetrated. If this were so, the direct proof that
egg-nucleus and sperm-nucleus are identical would be furnished.
Unfortunately the practical difficulties are so great that it is hardly
possible that the experiment can ever be made; but such want of
experimental proof is partially compensated for by the fact,
ascertained by Berthold, that in certain Algae (_Ectocarpus_ and
_Scytosiphon_) there is not only a female, but also a male
parthenogenesis; for he shows that in these species the male germ-cells
may sometimes develope into plants, which however are very weakly[175].
Furthermore the process of conjugation may be considered as a proof
that this view as to the secondary importance of sexual differentiation
is the true one. At the present time there can hardly be any hesitation
in accepting the view that conjugation is the sexual reproduction of
unicellular organisms. In these the two conjugating cells are almost
always identical in appearance, and there is no evidence in favour of
the assumption that they are not also identical in molecular structure,
at least so far as one individual of the same species may be identical
with another. But there are also forms in which the conjugating cells
are distinctly differentiated into male and female, and these are
connected with the former by a gradual transition: thus in _Pandorina_,
a genus of _Volvocineae_, we are unable to make out any differences
between the conjugating cells, while large egg-cells and minute
sperm-cells exist in the closely allied _Volvox_. If we must suppose
that the conjugation of two entirely identical Infusoria has the same
physiological effect as the union of two sexual cells in higher animals
and plants, we cannot escape the conclusion that the process is
essentially the same throughout: and that therefore the differences,
which are perhaps already indicated in _Pandorina_ and are very
distinct in _Volvox_ and in all higher organisms, have nothing to do
with the nature of the process, but are of quite secondary importance.
If we further take into account the extremely different constitution of
the two kinds of sexual cells in size, appearance, membranes, motile
power, and finally in number, no doubt remains that these differences
are only adaptations which secure the meeting of the two kinds of
conjugating cells: that in each species they are adaptations to the
peculiar conditions under which fertilization takes place.


                                 NOTE.

It is of considerable importance for the proper appreciation of the
views advanced in the present essay, to ascertain whether a polar body
is or is not expelled from eggs which develope parthenogenetically. I
wish therefore to briefly state that I have recently succeeded in
proving the formation of a polar body of distinctly cellular structure
in the summer-eggs of _Daphnidae_. I propose to publish a more detailed
account in a future paper.

                                                             A. W.

    _June 22, 1885._

------------------------------------------------------------------------


                        Footnotes for Essay IV.

Footnote 94:

  Häckel, ‘Ueber die Wellenzeugung der Lebenstheilchen etc.,’ Berlin,
  1876.

Footnote 95:

  Darwin, ‘The Variation of Animals and Plants under Domestication,’
  vol. ii. 1875, chap. xxvii. pp. 344-399.

Footnote 96:

  His, ‘Unsre Körperform etc.,’ Leipzig, 1875.

Footnote 97:

  Brooks, ‘The Law of Heredity,’ Baltimore, 1883.

Footnote 98:

  Galton’s experiments on transfusion in Rabbits have in the mean time
  really proved that Darwin’s gemmules do not exist. Roth indeed states
  that Darwin has never maintained that his gemmules make use of the
  circulation as a medium, but while on the one hand it cannot be shown
  why they should fail to take the favourable opportunities afforded by
  such a medium, inasmuch as they are said to be constantly circulating
  through the body; so on the other hand we cannot understand how the
  gemmules could contrive to avoid the circulation. Darwin has acted
  very wisely in avoiding any explanation of the exact course in which
  his gemmules circulate. He offered his hypothesis as a formal and not
  as a real explanation.

  Professor Meldola points out to me that Darwin did not admit that
  Galton’s experiments disproved pangenesis (‘Nature,’ April 27, 1871,
  p. 502), and Galton also admitted this in the next number of ‘Nature’
  (May 4, 1871, p. 5).—A. W. 1889.

Footnote 99:

  Weismann, ‘Ueber die Vererbung.’ Jena, 1883; translated in the
  present volume as the second essay ‘On Heredity.’

Footnote 100:

  E. Roth, ‘Die Thatsachen der Vererbung.’ 2. Aufl., Berlin, 1885, p.
  14.

Footnote 101:

  Jäger, ‘Lehrbuch der allgemeinen Zoologie,’ Bd. II. Leipzig, 1878.

Footnote 102:

  M. Nussbaum, ‘Die Differenzirung des Geschlechts im Thierreich,’
  Arch. f. Mikrosk. Anat., Bd. XVIII. 1880.

Footnote 103:

  I have since learnt that Professor Rauber of Dorpat also expressed
  similar views in 1880; and Professor Herdman of Liverpool informs me
  that Mr. Francis Galton had brought forward in 1876 a theory of
  heredity of which the fundamental idea in some ways approached that
  of the continuity of the germ-plasm (‘Journal of the Anthropological
  Institute,’ vol. v; London, 1876).—A. W., 1888.

  [A less complete theory was brought forward by Galton at an earlier
  date, in 1872 (see Proc. Roy. Soc. No. 136, p. 394). In this paper he
  proposed the idea that heredity chiefly depends upon the development
  of the offspring from elements directly derived from the fertilized
  ovum which had produced the parent. Galton speaks of the fact that
  ‘each individual may properly be conceived as consisting of two
  parts, one of which is latent and only known to us by its effects on
  his posterity, while the other is patent, and constitutes the person
  manifest to our senses. The adjacent and, in a broad sense, separate
  lines of growth in which the patent and latent elements are situated,
  diverge from a common group and converge to a common contribution,
  because they were both evolved out of elements contained in a
  structureless ovum, and they, jointly, contribute the elements which
  form the structureless ova of their offspring.’ The following diagram
  shows clearly ‘that the span of each of the links in the general
  chain of heredity extends from one structureless stage to another,
  and not from person to person:—

  Structureless elements {...Adult Father...    } structureless elements
       in Father         {...Latent in Father...} in Offspring.’

  Again Galton states—‘Out of the structureless ovum the embryonic
  elements are taken ... and these are developed (_a_) into the visible
  adult individual; on the other hand ..., after the embryonic elements
  have been segregated, the large residue is developed (_b_) into the
  latent elements contained in the adult individual.’ The above quoted
  sentences and diagram indicate that Galton does not derive the whole
  of the hereditary tendencies from the latent elements, but that he
  believes some effect is also produced by the patent elements. When
  however he contrasts the relative power of these two influences, he
  attaches comparatively little importance to the patent elements. Thus
  if any character be fixed upon, Galton states that it ‘may be
  conceived (1) as purely personal, without the concurrence of any
  latent equivalents, (2) as personal but conjoined with latent
  equivalents, and (3) as existent wholly in a latent form.’ He argues
  that the hereditary power in the first case is exceedingly feeble,
  because ‘the effects of the use and disuse of limbs, and those of
  habit, are transmitted to posterity in only a very slight degree.’ He
  also argues that many instances of the supposed transmission of
  personal characters are really due to latent equivalents. ‘The
  personal manifestation is, on the average, though it need not be so
  in every case, a certain proof of the existence of latent elements.’
  Having argued that the strength of the latter in heredity is further
  supported by the facts of reversion, Galton considers it is safe to
  conclude ‘that the contribution from the patent elements is very much
  less than from the latent ones.’ In the later development of his
  theory, Galton adheres to the conception of ‘gemmules’ and accepts
  Darwin’s views, although ‘with considerable modification.’ Together
  with pangenesis itself, Galton’s theory must be looked upon as
  _preformational_, and so far it is in opposition to Weismann’s theory
  which is _epigenetic_. See Appendix IV. to the next Essay (V.), pp.
  316-319.—E. B. P.]

Footnote 104:

  Nägeli, ‘Mechanisch-physiologische Theorie der Abstammungslehre.’
  München u. Leipzig, 1884.

Footnote 105:

  O. Hertwig, ‘Beiträge zur Kenntniss der Bildung, Befruchtung und
  Theilung des thierischen Eies.’ Leipzig, 1876.

Footnote 106:

  Fol, ‘Recherches sur la fécondation, etc.’ Genève, 1879.

Footnote 107:

  Kölliker formerly stated, and has again repeated in his most recent
  publication, that the spermatozoa (‘Samenfäden’) are mere nuclei. At
  the same time he recognizes the existence of sperm-cells in certain
  species. But proofs of the former assertion ought to be much stronger
  in order to be sufficient to support so improbable a hypothesis as
  that the elements of fertilization may possess a varying
  morphological value. Compare Zeitschr. f. wiss. Zool., Bd. XLII.

Footnote 108:

  F. M. Balfour, ‘Comparative Embryology,’ vol. i. p. 69.

Footnote 109:

  Arch. f. mikr. Anat., Bd. 23. p. 182, 1884.

Footnote 110:

  Born, ‘Biologische Untersuchungen,’ I, Arch. Mikr. Anat., Bd. XXIV.

Footnote 111:

  Roux, ‘Beiträge zum Entwicklungsmechanismus des Embryo,’ 1884.

Footnote 112:

  O. Hertwig, ‘Welchen Einfluss übt die Schwerkraft,’ etc. Jena, 1884.

Footnote 113:

  [Our present knowledge of the development of vegetable ova (including
  the position of the parts of the embryo) is also in favour of the
  view that it is not influenced by external causes, such as
  gravitation and light. It takes place in a manner characteristic of
  the genus or species, and essentially depends on other causes which
  are fixed by heredity, see Heinricher ‘Beeinflusst das Licht die
  Organanlage am Farnembryo?’ in Mittheilungen aus dem Botanischen
  Institute zu Graz, II. Jena, 1888.—S. S.]

Footnote 114:

  E. van Beneden, ‘Recherches sur la maturation de l’œuf,’ etc., 1883.

Footnote 115:

  M. Nussbaum, ‘Ueber die Veränderung der Geschlechtsprodukte bis zur
  Eifurchung,’ Arch. Mikr. Anat., 1884.

Footnote 116:

  Eduard Strasburger, ‘Neue Untersuchungen über den Befruchtungsvorgang
  bei den Phanerogamen als Grundlage für eine Theorie der Zeugung.’
  Jena, 1884.

  [It is now generally admitted that, in the Vascular Cryptogams, as
  also in Mosses and Liverworts, the bodies of the spermatozoids are
  formed by the nuclei of the cells from which they arise. Only the
  cilia which they possess, and which obviously merely serve as
  locomotive organs, are said to arise from the surrounding cytoplasm.
  It is therefore in these plants also the nucleus of the male cell
  which effects the fertilization of the ovum. See Göbel, ‘Outlines of
  Classification and Special Morphology,’ translated by H. E. F.
  Garnsey, edited by I. B. Balfour, Oxford, 1887, p. 203, and Douglas
  H. Campbell, ‘Zur Entwicklungsgeschichte der Spermatozoiden,’ in
  Berichte d. deutschen bot. Gesellschaft, vol. v (1887), p. 120.—S. S.]

Footnote 117:

   O. Hertwig, ‘Das Problem der Befruchtung und der Isotropie des
  Eies.’ Jena, 1885.

Footnote 118:

  This opinion was first expressed in my lecture, ‘Ueber die Dauer des
  Lebens,’ Jena, 1882, translated as the first essay in the present
  volume.

Footnote 119:

  M. Nussbaum, ‘Sitzungber. der Niederrheinischen Gesellschaft fur
  Natur- und Heilkunde.’ Dec. 15, 1884.

Footnote 120:

  A. Gruber, ‘Biologisches Centralblatt,’ Bd. IV. No. 23, and V. No. 5.

Footnote 121:

  According to the observations of Nussbaum and van Beneden, the egg of
  _Ascaris_ departs from the ordinary type, but I think that the latter
  observer goes too far when he concludes from the form of the nuclear
  spindle (of which the two halves are inclined to each other at an
  angle) that we have before us a process entirely different from that
  of ordinary nuclear division.

Footnote 122:

  Trinchese, ‘I primi momenti dell’ evoluzione nei molluschi,’ Atti
  Acad. Lyncei (3) vii. 1879, Roma.

Footnote 123:

  M. Nussbaum, ‘Archiv für Mikroskopische Anatomie,’ Bd. XVIII und
  XXIII.

Footnote 124:

  Valaoritis, ‘Die Genesis des Thier-Eies.’ Leipzig, 1882.

Footnote 125:

  Kölliker, ‘Die Bedeutung der Zellkerne,’ etc.; Zeitschr. f. wiss.
  Zool. Bd. XLII.

Footnote 126:

  ‘Compt. rend.’ Tom. LIV. p. 150.

Footnote 127:

  ‘Entwicklung der Dipteren.’ Leipzig, 1864.

Footnote 128:

  ‘Zeitschr. f. wiss. Zool.’ Bd. XVI. p. 389 (1866).

Footnote 129:

  ‘Compt. rend.’ Nov. 13, 1882.

Footnote 130:

  Grobben, ‘Arbeiten d. Wien. Zool. Instituts,’ Bd. II. p. 203.

Footnote 131:

  Bütschli, ‘Zeitschrift f. wiss. Zool.’ Bd. XXIII. p. 409.

Footnote 132:

  ‘Science,’ vol. iv. No. 90, 1884.

Footnote 133:

  Among unicellular organisms, encysted individuals are often called
  germs. They sometimes differ from the adult organism in their smaller
  size and simpler structure (_Gregarinidae_), but they represent the
  same morphological stage of individuality.

Footnote 134:

  Compare Bütschli in Bronn’s ‘Klassen und Ordnungen des Thierreichs,’
  Bd. I. p. 777.

Footnote 135:

  Gustav Jäger, ‘Lehrbuch der Allgemeinen Zoologie,’ Leipzig, 1878; II.
  Abtheilung. Probably on account of the extravagant and superficial
  speculations of the author, the valuable ideas contained in his book
  have been generally overlooked. It is only lately that I have become
  aware of Jäger’s above-mentioned hypothesis. M. Nussbaum seems to
  have also arrived at the same conclusion quite independently of
  Jäger. The latter has not attempted to work out his hypothesis with
  any degree of completeness. The above-mentioned observations are
  followed immediately by quite valueless considerations, as, for
  instance, that the ontogenetic and phyletic groups are in concentric
  ratio! The author might as well speak of a quadrangular or triangular
  ratio!

Footnote 136:

  [Facts of the same kind are also known in the Vascular Cryptogams,
  Muscineae, Characeae, Florideae, etc.—S. S.]

Footnote 137:

  Weismann, ‘Die Entstehung der Sexualzellen bei den Hydromedusen.’
  Jena, 1883.

Footnote 138:

  [I adopt this term, suggested by E. Ray Lankester and G. C. Bourne,
  as the name of the supporting lamina of Coelenterata. See ‘Quart.
  Journ. Microsc. Sci.’ Jan. 1887, p. 28.—E. B. P.]

Footnote 139:

  Dr. Clemens Hartlaub, ‘Ueber die Entstehung der Sexualzellen bei
  Obelia.’ Freiburg, Inaugural Dissertation: see also ‘Zeitschrift für
  wissenschaftliche Zoologie.’ Bd. XLI. 1884.

Footnote 140:

   English translation, by H. Marshall Ward. Oxford, 1887, Clarendon
  Press.

Footnote 141:

  [Such gland-cells are known in both animals and plants. See W.
  Gardiner and Tokutaro Ito, On the structure of the mucilage-secreting
  cells of _Blechnum occidentale_ L., and _Osmunda regalis_ L., ‘Annals
  of Botany,’ vol. i. p. 49.—S. S.]

Footnote 142:

  Thus in 1877 Bütschli thought that ‘the chief significance of the
  formation of polar bodies lies in the removal of part of the nucleus
  of the egg, whether this removal is effected by simple expulsion or
  by the budding of the egg-cell.’ ‘Entwicklungsgeschichtliche
  Beiträge;’ Zeitschrift für wissenschaftliche Zoologie, Bd. XXIX. p.
  237, footnote.

Footnote 143:

  C. S. Minot, ‘Account, etc.;’ Proc. Boston Soc. Nat. Hist. vol. xix.
  p. 165, 1877.

Footnote 144:

  E. van Beneden and Boveri have recently, quite independently of each
  other, made a more exact study of these ‘Polkörperchen’
  (‘Centrosoma,’ Boveri). They show that nuclear division starts from
  these bodies, although the mode of origin of the latter is not yet
  quite clear.—A. W., 1888.

Footnote 145:

  The existence of polar bodies in sponges has been recently proved by
  Fiedler: Zool., Anzeiger., Nov. 28, 1887.—A. W., 1888.

Footnote 146:

  They have now been observed in many species, so that their general
  occurrence in insects is tolerably certain. Compare bibliography
  given in Weismann and Ischikawa, ‘Weitere Untersuchungen zum
  Zahlengesetz der Richtungskörper,’ ‘Zoolog. Jahrbücher,’ vol. iii.
  1888, p. 593.—A. W., 1888.

Footnote 147:

  Van Beneden, even in his last work, considers these bodies to have
  only the value of nuclei; l. c., p. 394.

Footnote 148:

  I purposely abstain from using a more precise term, for the
  complicated terminology employed in spermatogenesis hardly
  contributes anything to the elucidation of the phenomena themselves.
  Why do we not simply speak of sperm-cells and spermatoblasts, and
  distinguish the latter by numbers when they occur in successive
  generations of different form? Moreover, all the names which have
  been suggested for successive stages of development, can only be
  applied to the special group of animals upon which the observations
  have been made. Hence great confusion results from the use of such
  terms as spermatoblasts, spermatogonia, spermatomeres, spermatocysts,
  spermatocytes, spermatogemmae, etc.

Footnote 149:

  Fol, ‘Sur l’origine des cellules du follicule et de l’ovule chez les
  Ascidies.’ Compt. rend., 28 mai, 1883.

Footnote 150:

  Roule, ‘La structure de l’ovaire et la formation des œufs chez les
  Phallusiadées.’ Ibid., 9 avril, 1883.

Footnote 151:

  Balbiani, ‘Sur l’origine des cellules du follicule et du noyau
  vitellin de l’œuf chez les Géophiles.’ Zool. Anzeiger, 1883, Nos.
  155, 156.

Footnote 152:

  Will, ‘Ueber die Entstehung des Dotters und der Epithelzellen bei den
  Amphibien und Insecten.’ Ibid., 1884, Nos. 167, 168.

Footnote 153:

  [It is almost certain that this vesicle is not derived from the
  nucleus, but from the cytoplasm of the sperm-mother-cell. See Douglas
  H. Campbell, ‘Zur Entwicklungsgeschichte der Spermatozoiden’ in
  Berichte der deutschen botanischen Gesellschaft, vol. v, 1887, p.
  122.—S. S.]

Footnote 154:

  Bütschli, ‘Gedanken über die morphologische Bedeutung der sogenannten
  Richtungskörperchen,’ Biolog. Centralblatt, Bd. VI. p. 5, 1884.

Footnote 155:

  F. M. Balfour, ‘Comparative Embryology,’ vol. i. p. 63.

Footnote 156:

  The formation of a polar body in parthenogenetic eggs has now been
  proved: see note at the end of this Essay; see also Essay VI.—A. W.,
  1888.

Footnote 157:

   R. Leuckart,—article ‘Zeugung,’ in R. Wagner’s ‘Handwörterbuch der
  Physiologie,’ 1853, Bd. IV. p. 958. Similar observations were made by
  Max Schultze. These observations appear however to be erroneous, for
  Pflüger has since shown that the eggs of frogs never develope if the
  necessary precautions are taken to prevent the access of any
  spermatozoa to the water.—A. W., 1888.

Footnote 158:

  Oellacher, ‘Die Veränderungen des unbefruchteten Keims des
  Hühncheneies. ‘Zeitschrift für wissenschaftliche Zoologie,’ Bd. XXII.
  p. 181. 1872.

Footnote 159:

  Hensen, ‘Centralblatt,’ 1869, No. 26.

Footnote 160:

  Weismann, ‘Beiträge zur Naturgeschichte der Daphnoiden,’ Leipzig,
  1876-79, Abhandlung VII, and ‘Zeitschrift für wissenschaftliche
  Zoologie,’ Bd. XXXIII.

Footnote 161:

  Weismann, ‘Beiträge zur Kenntniss der ersten Entwicklungsvorgänge im
  Insectenei,’ Bonn, 1882, p. 106.

Footnote 162:

   W. Roux, ‘Ueber die Bedeutung der Kerntheilungsfiguren.’ Leipzig,
  1883.

Footnote 163:

  We now know that the number of loops varies considerably in different
  species, even when they belong to the same group of animals (e.g.
  Nematodes).—A.W., 1888.

Footnote 164:

  This expression is used by bee-keepers, for instance by the
  well-known Baron Berlepsch. Of course, it would be more accurate to
  say that the queen, seeing the cell of a drone, is stimulated to lay
  an unfertilized egg, and that, on the other hand, she is stimulated
  to lay a fertilized egg when she sees the cell of a worker, or that
  of a queen.

Footnote 165:

  E. Bessels, ‘Die Landois’sche Theorie widerlegt durch das
  Experiment.’ Zeitschrift für wissenschaftliche Zoologie, Bd. XVIII.
  p. 124. 1868.

Footnote 166:

  ‘Daphniden,’ Abhandlung, vi. p. 324.

Footnote 167:

  l. c., p. 150.

Footnote 168:

  Carl Düsing, ‘Die Regulirung des Geschlechtsverhältnisses.’ Jena.
  1884.

Footnote 169:

  I intend to publish these experiments elsewhere in connexion with
  other observations.

Footnote 170:

  Weismann, ‘Daphniden,’ Abhandlung, VII. p. 329; Herbert Spencer, ‘The
  Principles of Biology,’ 1864, vol. i. pp. 229, 230.

Footnote 171:

  The same fact has since been ascertained in species belonging to
  several groups of animal.

Footnote 172:

  Brooks, ‘The Law of Heredity.’ Baltimore, 1883, p. 73.

Footnote 173:

  ‘Zeitschrift für wissenschaftliche Zoologie,’ Bd. XXXIII. p. 107.
  1873.

Footnote 174:

  Valaoritis, l. c., p. 6.

Footnote 175:

  I quote from Falkenberg, in Schenk’s ‘Handbuch der Botanik,’ Bd. II.
  p. 219. He further states that these are the only instances hitherto
  known in which undoubted male cells have proved to be capable of
  further development when they have been unable to exercise their
  powers of fertilization. It must be added that the two kinds of
  germ-cells do not differ in appearance, but only in behaviour; the
  female germ-cells becoming fixed, and withdrawing one of their two
  flagella, while the male cells continue to swarm. But even this
  slight degree of differentiation requires the supposition of internal
  molecular differentiation.

------------------------------------------------------------------------






                                   V.

                THE SIGNIFICANCE OF SEXUAL REPRODUCTION
                  IN THE THEORY OF NATURAL SELECTION.

                                 1886.




------------------------------------------------------------------------




               SIGNIFICANCE OF SEXUAL REPRODUCTION, ETC.

                                PREFACE.

The greater part of the present essay was delivered at the first
general meeting of the Association of German Naturalists, at
Strassburg, on September 18th, 1885, and is printed in the Proceedings
of the fifty-eighth meeting of that Society.

The form of a lecture has been retained in the present publication, but
its contents have been extended in many ways. Besides many small and a
few large additions to the text, I have added six appendices in order
to treat of certain subjects more fully than was possible in the
lecture itself, in which I was often obliged to be content with mere
hints and suggestions. This appears to be all the more necessary
because it is impossible to suppose that many views and ideas upon
which the lecture was based would be well known to all readers,
although they have been described in my former papers. It was above all
necessary to deal with the class of acquired characters, which, as it
seems to me, is easily confounded, especially by the medical
profession, with the much broader class of new characters generally.
Only those new characters can be called ‘acquired’ which owe their
origin to external influences, and the term ‘acquired’ must be denied
to those which depend upon the mysterious relationship between the
different hereditary tendencies which meet in the fertilized ovum.
These latter are not ‘acquired’ but inherited, although the ancestors
did not possess them as such, but only as it were the elements of which
they are composed. Such new characters as these do not at present admit
of an exact analysis: we have to be satisfied with the undoubted fact
of their occurrence. The transmission or non-transmission of acquired
characters must be of the highest importance for a theory of heredity,
and therefore for the true appreciation of the causes which lead to the
transformation of species. Any one who believes, as I do, that acquired
characters are not transmitted, will be compelled to assume that the
process of natural selection has had a far larger share in the
transformation of species than has been as yet accorded to it; for if
such characters are not transmitted, the modifying influence of
external circumstances in many cases remains restricted to the
individual, and cannot have any part in producing transformation. We
shall also be compelled to abandon the ideas as to the origin of
individual variability which have been hitherto accepted, and shall be
obliged to look for a new source of this phenomenon, upon which the
processes of selection entirely depend.

In the following pages I have attempted to suggest such a source.

                                                             A. W.

      Freiburg I. Br.,

          _November 22, 1885._




------------------------------------------------------------------------

               SIGNIFICANCE OF SEXUAL REPRODUCTION, etc.

                               CONTENTS.

                                                               PAGE

       1. Can we dispense with the principle of natural         255
          selection?

       2. Nägeli’s theory of transformation from internal       256
          causes

       3. A definite course of development is possible          258
          without a self-changing idioplasm

       4. Conclusive importance of ‘adaptations’                260

       5. The structure of whales as an example of              261
          adaptation

       6. Transformation takes place by the smallest steps      264

       7. The foundation of such minute changes depends upon    266
          individual variability

       8. Difficulty in accounting for variability on the       266
          supposition of a continuity of the germ-plasm

       9. Previous theories by which variability has been       267
          accounted for

      10. Non-transmission of acquired characters               267

      11. Nägeli’s and Alexis Jordan’s experiments              269

      12. Germ-plasm is only altered with great difficulty      271

      13. The source of individual variation lies in sexual     272
          reproduction

      14. The process of natural selection does not operate     274
          when asexual reproduction takes place

      15. Origin of variability in unicellular organisms        278

      16. Sexual reproduction effects combination               279

      17. E. van Beneden’s and V. Hensen’s theory of sexual     282
          reproduction as a process of rejuvenescence

      18. Theoretical objections to such a view                 283

      19. Original significance of conjugation                  286

      20. Preservation of sexual reproduction by means of       287
          heredity

      21. It is lost in parthenogenesis for reasons of          289
          utility

      22. Parthenogenesis prevents further transformations      290

      23. It excludes Panmixia and thus prevents disused        291
          organs from becoming rudimentary

      24. Final considerations                                  294


                              APPENDICES.

        I. Further considerations which oppose Nägeli’s         298
           explanation of Transformation as due to internal
           causes

       II. Nägeli’s Explanation of Adaptation                   300

      III. Adaptations in Plants                                308

       IV. On the Supposed Transmission of Acquired             310
           Characters

           1. Brown-Séquard’s experiments on Guinea-pigs        310

           2. A case which at first sight appears to prove      320
           the transmission of acquired characters

        V. On the Origin of Parthenogenesis                     323

       VI. W. K. Brooks’ Theory of Heredity                     326




------------------------------------------------------------------------

                                   V.

                THE SIGNIFICANCE OF SEXUAL REPRODUCTION
                  IN THE THEORY OF NATURAL SELECTION.


During the quarter of a century which has elapsed since Biology began
to occupy itself again with general problems, at least one main fact
has been made clear by the united labours of numerous men of science,
viz. the fact that the Theory of Descent, the idea of development in
the organic world, is the only conception as to the origin of the
latter, which is scientifically tenable. It is not only that, in the
light of this theory, numerous facts receive for the first time a
meaning and significance; it is not only that, under its influence, all
the ascertained facts can be harmoniously grouped together; but in some
departments it has already yielded the highest results which can be
expected from any theory, it has rendered possible the prediction of
facts, not indeed with the absolute certainty of calculation, but still
with a high degree of probability. It has been predicted that man, who,
in the adult state, only possesses twelve pairs of ribs, would be found
to have thirteen or fourteen in the embryonic state: it has been
predicted that, at this early period in his existence, he would possess
the insignificant remnant of a very small bone in the wrist, the
so-called _os centrale_, which must have existed in the adult condition
of his extremely remote ancestors. Both predictions have been
fulfilled, just as the planet Neptune was discovered after its
existence had been predicted from the disturbances induced in the orbit
of Uranus.

That existing species have not arisen independently, but have been
derived from other and mostly extinct species, and that on the whole
this development has taken place in the direction of greater
complexity, may be maintained with the same degree of certainty as that
with which astronomy asserts that the earth moves round the sun; for a
conclusion may be arrived at as safely by other methods as by
mathematical calculation.

If I make this assertion so unhesitatingly, I do not make it in the
belief that I am bringing forward anything new nor because I think that
any opposition will be encountered, but simply because I wish to begin
by pointing out the firm ground on which we stand, before considering
the numerous problems which still remain unsolved. Such problems appear
as soon as we pass from the facts of the case to their explanation; as
soon as we pass from the statement ‘The organic world has arisen by
development,’ to the question ‘But how has this been effected, by the
action of what forces, by what means, and under what circumstances?’

In attempting to answer these questions we are very far from dealing
with certainties; and opinions are still conflicting. But the answer
lies in the domain of future investigation, that unknown country which
we have to explore.

It is true that this country is not entirely unknown, and if I am not
mistaken, Charles Darwin, who in our time has been the first to revive
the long-dormant theory of descent, has already given a sketch, which
may well serve as a basis for the complete map of the domain; although
perhaps many details will be added, and many others taken away. In the
principle of natural selection, Darwin has indicated the route by which
we must enter this unknown land.

But this opinion is not universal, and only recently Carl Nägeli[176],
the famous botanist, has expressed decided doubts as to the general
applicability of the principle of natural selection. According to
Nägeli, the co-operation of the external conditions of life with the
known forces of the organism, viz. heredity and variability, are
insufficient to explain the regular course of development pursued by
the organic world. He considers that natural selection is at best an
auxiliary principle, which accepts or rejects existing characters, but
which is unable to create anything new: he believes that the causes of
transformation reside within the organism alone. Nägeli further assumes
that organisms contain forces which cause periodical transformation of
the species, and he imagines that the organic world, as a whole, has
arisen in a manner similar to that in which a single individual arises.

Just as a seed produces a certain plant because it possesses a certain
constitution, and just as, in this process, certain conditions must be
favourable (light, warmth, moisture, &c.) in order that development may
take place, although they do not determine the kind or the manner of
development; so, in precisely the same way, the tree of the whole
organic world has grown up from the first and lowest forms of life on
our planet, under a necessity arising from within, and on the whole
independently of external influences. According to Nägeli, the cause
which compels every form of living substance to change, from time to
time, in the course of its secular growth, and which moulds it afresh
into new species, must lie within the organic substance itself, and
must depend upon its molecular structure.

It is with sincere admiration and real pleasure that we read the
exposition in which Nägeli gives, as it were, the result of all his
researches which bear upon the great question of the development of the
organic world. But although we derive true enjoyment from the
contemplation of the elaborate and ingeniously wrought-out theoretical
conception,—which like a beautiful building or a work of art is
complete in itself,—and although we must be convinced that its rise has
depended upon the progress of knowledge, and that by its means we shall
eventually reach a fuller knowledge; it is nevertheless true that we
cannot accept the author’s fundamental hypothesis. I at least believe
that I am not alone in this respect, and that but few zoologists will
be found who can adopt the hypothesis which forms the foundation of
Nägeli’s theory.

It is not my intention at present to justify my own widely different
views, but the subject of this lecture compels me to briefly explain my
position in relation to Nägeli, and to give some of the reasons why I
cannot accept his theory of an active force of transformation arising
and working within the organism; and I must also explain the reasons
which induce me to adhere to the theory of natural selection.

The supposition of such a phyletic force of transformation (see
Appendix I, p.298) possesses, in my opinion, the greatest defect that
any theory can have,—it does not explain the phenomena. I do not mean
to imply that it is incapable of rendering certain subordinate
phenomena intelligible, but that it leaves a larger number of facts
entirely unexplained. It does not afford any explanation of the
purposefulness seen in organisms: and this is just the main problem
which the organic world offers for our solution. That species are, from
time to time, transformed into new ones might perhaps be understood by
means of an internal transforming force, but that they are so changed
as to become better adapted to the new conditions under which they have
to live, is left entirely unintelligible by this theory. For we
certainly cannot accept as an explanation Nägeli’s statement that
organisms possess the power of being transformed in an adaptive manner
simply by the action of an external stimulus (see Appendix II, p. 300).

In addition to this fundamental defect, we must also note that there
are absolutely no proofs in support of the foundation of this theory,
viz. of the existence of an internal transforming force.

Nägeli has very ingeniously worked out his conception of idioplasm, and
this conception is certainly an important acquisition and one that will
last, although without the special meaning given to it by its author.
But is this special meaning anything more than pure hypothesis? Can we
say more than this of the ingenious description of the minute molecular
structure of the hypothetical basis of life? Could not idioplasm be
built up in a manner entirely different from that which Nägeli
supposes? And can conclusions drawn from its supposed structure be
brought forward to prove anything? The only proof that idioplasm must
necessarily change, in the course of time, as the result of its own
structure, is to be found in the fact that Nägeli has so constructed
it; and no one will doubt that the structure of idioplasm might have
been so conceived as to render any transformation from within itself
entirely impossible.

But even if it is theoretically possible to imagine that idioplasm
possesses such a structure that it changes in a certain manner, as the
result of mere growth, we should not be justified in thus assuming the
existence of a new and totally unknown principle until it had been
proved that known forces are insufficient for the explanation of the
observed phenomena.

Can any one assert that this proof has been forthcoming? It has been
again and again pointed out that the phyletic development of the
vegetable kingdom proceeds with regularity and according to law, as we
see in the preponderance and constancy of so-called purely
‘morphological’ characters in plants. The formation of natural groups
in the animal and vegetable kingdoms compels us to admit that organic
evolution has frequently proceeded for longer or shorter periods along
certain developmental lines. But we are not on this account compelled
to adopt the supposition of unknown internal forces which have
determined such lines of development.

Many years ago I attempted to prove[177] that the constitution or
physical nature of an organism must exercise a restricting influence
upon its capacity for variation. A given species cannot change into any
other species, which may be thought of. A beetle could not be
transformed into a vertebrate animal: it could not even become a
grasshopper or a butterfly; but it could change into a new species of
beetle, although only at first into a species of the same genus. Every
new species must have been directly continuous with the old one from
which it arose, and this fact alone implies that phyletic development
must necessarily follow certain lines.

I can fully understand how it is that a botanist has more inclination
than a zoologist to take refuge in internal developmental forces. The
relation of form to function, the adaptation of the organism to the
internal and external conditions of life, is less prominent in plants
than in animals; and it is even true that a large amount of observation
and ingenuity is often necessary in order to make out any adaptation at
all. The temptation to accept the view that everything depends upon
internal directing causes is therefore all the greater. Nägeli indeed
looks at the subject from the opposite point of view, and considers
that the true underlying cause of transformation is in animals obscured
by adaptation, but is more apparent in plants[178]. Sufficient
justification for this opinion cannot, however, be furnished by the
fact that in plants many characters have not been as yet explained by
adaptation. We should do well to remember the extent to which the
number of so-called ‘morphological’ characters in plants has been
lessened during the last twenty years. What a flood of light was thrown
upon the forms and colours of flowers, so often curious and apparently
arbitrary, when Sprengel’s long-neglected discovery was extended and
duly appreciated as the result of Darwin’s investigations, and when the
subject was further advanced by Hermann Müller’s admirable researches!
Even the venation of leaves, which was formerly considered to be
entirely without significance, has been shown to possess a high
biological value by the ingenious investigations of J. Sachs (see
Appendix III, p. 308). We have not yet reached the limits of
investigation, and no reason can be assigned for the belief that we
shall not some day receive an explanation of characters which are now
unintelligible[179].

It is obvious that the zoologist cannot lay too much stress upon the
intimate connexion between form and function, a connexion which extends
to the minutest details: it is almost impossible to insist too much
upon the perfect manner in which adaptation to certain conditions of
life is carried out in the animal body. In the animal body we find
nothing without a meaning, nothing which might be otherwise; each
organ, even each cell or part of a cell is, as it were, tuned for the
special part it has to perform in relation to the surroundings.

It is true that we are as yet unable to explain the adaptive character
of every structure in any single species, but whenever we succeed in
making out the significance of a structure, it always proves to be a
fresh example of adaptation. Any one who has attempted to study the
structure of a species in detail, and to account for the relation of
its parts to the functions of the whole, will be altogether inclined to
believe with me that everything depends upon adaptation. There is no
part of the body of an individual or of any of its ancestors, not even
the minutest and most insignificant part, which has arisen in any other
way than under the influence of the conditions of life; and the parts
of the body conform to these conditions, as the channel of a river is
shaped by the stream which flows over it.

These are indeed only convictions, not real proofs; for we are not yet
sufficiently intimately acquainted with any species to be able to
recognize the nature and meaning of all the details of its structure,
in all their relations: and we are still less able to trace the
ancestral history in each case, and to make out the origin of those
structures of which the presence in the descendants depends primarily
upon heredity. But already a fair advance towards the attainment of
inductive proof has been made; for the number of adaptations which have
been established is now very large and is increasing every day. If,
however, we anticipate the results of future researches, and admit that
an organism only consists of adaptations, based upon an ancestral
constitution, it is obvious that nothing remains to be explained by a
phyletic force, even though the latter be presented to us in the
refined form of Nägeli’s self-changing idioplasm.

It will perhaps be useful to illustrate my views by a familiar example.
I choose the well-known group of the whales. These animals are
placental mammals, which, probably in secondary times, arose from
terrestrial Mammalia, by adaptation to an aquatic life.

Everything that is characteristic of these animals and distinguishes
them from other mammals depends upon this adaptation. Their fore-limbs
have been transformed into rigid paddles, only movable at the
shoulder-joint; upon the back and the tail there are ridges with a form
somewhat similar to the dorsal and caudal fins of fishes. The organ of
hearing is without any external ear and without an air-containing
external auditory meatus. The aerial vibrations do not pass, as in
other mammals, from the external auditory passage to the tympanic
cavity and thus to the nerve-terminations of the inner ear; but they
reach the tympanic cavity by direct transmission through the bones of
the skull, which possess a special structure and contain abundant
air-cavities. This arrangement is obviously adapted for hearing in
water. The nostrils also exhibit peculiarities, for they do not open
near the mouth, but upon the forehead, so that the animal can breathe,
even in a rough sea, as soon as it comes to the surface. In order to
facilitate rapid movement in water, the whole body has become extended
in length, and spindle-shaped, like the body of a fish. The hind limbs
are absent in no other mammals, the fish-like _Sirenia_ being alone
excepted. In the whales, as in the _Sirenia_, these appendages have
become useless, owing to the powerfully developed tail-fin; they are
now rudimentary and consist of some small bones and muscles deeply
buried in the body of the animal, which nevertheless, in certain
species, still exhibit the original structure of the hind-limb. The
hairy covering of other mammals has also disappeared, its place having
been taken by a thick layer of fat beneath the skin, which affords a
much better protection against cold. This fatty layer was also
necessary in order to diminish the specific gravity of the animal, and
to thus render it equal to that of sea-water. In the structure of the
skull there are also a number of peculiarities, all of which are
directly or indirectly connected with the conditions under which these
animals live. In the whalebone whales, the enormous size of the face,
the immense jaws, and wide mouth are very striking. Can it be suggested
that this very characteristic appearance is entirely due to the
guidance of some internal transforming force, or to some spontaneous
modification of the idioplasm? Any such suggestion cannot be accepted,
for it is easy to show that all these structural features depend upon
adaptation to a peculiar mode of feeding. Functional teeth are absent,
but rudimentary ones exist in the embryo as relics of an ancestral
condition in which these organs were fully developed. Large plates of
whalebone with finely divided ends are suspended vertically from the
roof of the mouth. These whales feed upon small organisms, about an
inch in length, which swim or float upon the water in countless
numbers; and in order that they may subsist upon such minute animals,
it is necessary to obtain them in immense numbers. This is achieved by
means of the huge mouth which takes in a vast quantity of water at a
single mouthful. The water then filters away through the plates of
whalebone, while the organisms which form the whale’s food remain
stranded in the mouth. Is it necessary to add that the internal
organs—so far as we understand the details of their functions, and so
far as their structure differs from that of the corresponding organs in
other Mammalia—have also been directly or indirectly modified by
adaptation to an aquatic life? Thus all whales possess a very peculiar
arrangement of the nasal passages and larynx, enabling them to breathe
and swallow at the same time: the lungs are of enormous length, and
thus cause the animal to assume a horizontal position in the water
without the exercise of muscular effort: in consequence of this latter
modification, the diaphragm extends in a nearly horizontal direction:
there are moreover certain arrangements in the vascular system which
enable the animal to remain under water for a considerable time, and so
on.

And now, in reference to this special example, I will repeat the
question which I have asked before:—‘If everything that is
characteristic of a group of animals depends upon adaptation, what
remains to be explained by the operation of an internal developmental
force?’ What remains of a whale when we have taken away its adaptive
characters? We are compelled to reply that nothing remains except the
general plan of mammalian organization, which existed previously in the
mammalian ancestors of the _Cetacea_. But if everything which stamps
these animals as whales has arisen by adaptation, it follows that the
internal developmental force cannot have had any share in the origin of
this group.

And yet this very force is said to be the main factor in the
transformation of species, and Nägeli unhesitatingly asserts that both
the animal and vegetable kingdoms would have become very much as they
now are, if there had been no adaptation to new conditions, and no such
thing as competition in the struggle for existence[180].

But even if we admit that such an assumption affords some explanation,
instead of being the renunciation of all attempts at explanation; if we
admit that an organism, the characteristic peculiarities of which
entirely depend upon adaptation, has been formed by an internal
developmental force; we should still be unable to explain how it
happens that such an organism, suited to certain conditions of life,
and unable to exist under other conditions, appeared at that very place
on the earth’s surface, and at that very time in the earth’s history,
which offered the conditions appropriate for its existence. As I have
previously argued, the believers in an internal developmental force are
compelled to invent an auxiliary hypothesis, a kind of ‘pre-established
harmony’ which explains how it is that changes in the organic world
advance step by step, parallel with changes in the crust of the earth
and in other conditions of life; just as, according to Leibnitz, body
and soul, although independent of each other, proceed along parallel
courses, like two chronometers which keep perfect time. And even this
supposition would not be sufficient, because the place must be taken
into account as well as the time: thus the whales could not have
existed if they had first appeared upon dry land. We know of countless
instances in which a species is exclusively and precisely adapted to a
certain localized area, and could not thrive anywhere else. We have
only to remember the cases of mimicry in which one insect gains
protection by resembling another, the cases of protective resemblance
to the bark or the leaves of a certain species of plant, or the
numerous marvellous adaptations of parasitic animals to certain parts
of certain species of hosts.

A mimetic species cannot have appeared at any place other than that in
which it exists: it cannot have arisen through an internal
developmental force. But if single species, or even whole orders like
the _Cetacea_, have arisen independently of any such force, then we may
safely assert that the existence of the supposed force is neither
required by reason nor necessity.

Hence, abstaining from the invocation of unknown forces, we are
justified in carrying on Darwin’s attempt to explain the transformation
of organisms by the action of known forces and known phenomena. I say
‘carry on the attempt,’ because I do not believe that our knowledge in
this direction has ended with Darwin, and it seems to me that we have
already arrived at ideas which are incompatible with certain important
points in his general theory, and which therefore necessitate some
modification of the latter.

The theory of natural selection explains the rise of new species by
supposing that changes occur, from time to time, in those conditions of
life to which an organism must adapt itself if it is to continue in
existence. Thus a selective process is set up which ensures that only
those out of the existing variations are preserved, which correspond in
the highest degree to the changed conditions of life. By continued
selection in the same direction the deviations from the type, although
at first very insignificant, are accumulated and increased until they
become specific differences.

I should wish to assert more definitely than Darwin has done, that
alterations in the conditions of life, together with changes in the
organism itself, must have advanced very gradually and by the smallest
steps, in such a way that, at each period in the whole process of
transformation, the species has remained sufficiently adapted to the
surrounding conditions. An abrupt transformation of a species is
inconceivable, because it would render the species incapable of
existence. If the whole organization of an animal depends upon
adaptation, if the animal body is, as it were, an extremely complex
combination of new and old adaptations, it would be a highly remarkable
coincidence if, after any sudden alteration occurring simultaneously in
many parts of the body, all these parts were changed in such a manner
that they again formed a whole which exactly corresponded to the
altered external conditions. Those who assume the existence of such a
sudden transformation overlook the fact that everything in the animal
body is exactly calculated to maintain the existence of the species,
and that it is just sufficient for this purpose; and they forget that
the minutest change in the least important organ may be enough to
render the species incapable of existence.

It may perhaps be objected that the case is different in plants, as is
proved by the American weeds which have spread all over Europe, or the
European plants which have become naturalized in Australia. Reference
might also be made to the plants which inhabited the plains during the
glacial epoch, and which at its close migrated to the Alpine mountains
and to the far north, and which have remained unaltered under the
apparently diverse conditions of life to which they have been subjected
for so long a time. Similar instances may also be found among animals.
The rabbit, which was brought by sailors to the Atlantic island of
Porto Santo, has bred abundantly and remains unchanged in this
locality; the European frogs, which were introduced into Madeira, have
increased immensely and have become almost a plague; and the European
sparrow now thrives in Australia quite as well as with us. But these
instances do not prove that adaptation to external conditions of life
is not of primary importance; they do not prove that an organism which
is adapted to a certain environment will, when unmodified, remain
capable of existence amid new surroundings. They only prove that the
above-mentioned species found in those countries the same conditions of
life as at home, or at least that they met with conditions to which
their organization could be subjected without the necessity for
modification. Not every new environment includes such changed
conditions as will be effective in modifying every species of plant or
animal. The rabbit of Porto Santo certainly feeds on herbs different
from those which form the food of its relations in Europe, but such a
change does not mean an effective alteration in the conditions under
which this species lives, for the herbs in both localities are equally
well suited to the needs of the animal.

But if we suppose that the wild rabbit, occurring in Europe, were to
suddenly lose but a trifle of its wariness, its keen sight, its fine
sense of hearing or of smell, or were to suddenly acquire a colour
different from that which it now possesses, it would become incapable
of existence as a species, and would soon die out. The same result
would probably occur if any of its internal organs, such as the lungs
or the liver, were suddenly modified. Perhaps single individuals would
still remain capable of existence under these circumstances, but the
whole species would suffer a certain decline from the maximum
development of its powers of resistance, and would thus become extinct.
The sudden transformation of a species appears to me to be
inconceivable from a physiological point of view, at any rate in
animals.

Hence the transformation of a species can only take place by the
smallest steps, and must depend upon the accumulation of those
differences which characterise individuals, or, as we call them,
‘individual differences.’ There is no doubt that these differences are
always present, and thus, at first sight, it appears to be simply a
matter of course that they will afford the material by means of which
natural selection produces new forms of life. But the case is not so
simple as it appeared to be until recently; that is if I am right in
believing that in all animals and plants which are reproduced by true
germs, only those characters which were potentially present in the germ
of the parent can be transmitted to the succeeding generation.

I believe that heredity depends upon the fact that a small portion of
the effective substance of the germ, the germ-plasm, remains unchanged
during the development of the ovum into an organism, and that this part
of the germ-plasm serves as a foundation from which the germ-cells of
the new organism are produced[181]. There is therefore continuity of
the germ-plasm from one generation to another. One might represent the
germ-plasm by the metaphor of a long creeping root-stock from which
plants arise at intervals, these latter representing the individuals of
successive generations.

Hence it follows that the transmission of acquired characters is an
impossibility, for if the germ-plasm is not formed anew in each
individual but is derived from that which preceded it, its structure,
and above all its molecular constitution, cannot depend upon the
individual in which it happens to occur, but such an individual only
forms, as it were, the nutritive soil at the expense of which the
germ-plasm grows, while the latter possessed its characteristic
structure from the beginning, viz. before the commencement of growth.

But the tendencies of heredity, of which the germ-plasm is the bearer,
depend upon this very molecular structure, and hence only those
characters can be transmitted through successive generations which have
been previously inherited, viz. those characters which were potentially
contained in the structure of the germ-plasm. It also follows that
those other characters which have been acquired by the influence of
special external conditions, during the life-time of the parent, cannot
be transmitted at all.

The opposite view has, up to the present time, been maintained, and it
has been assumed, as a matter of course, that acquired characters can
be transmitted; furthermore, extremely complicated and artificial
theories have been constructed in order to explain how it may be
possible for changes produced by the action of external influences, in
the course of a life-time, to be communicated to the germ and thus to
become hereditary. But no single fact is known which really proves that
acquired characters can be transmitted, for the ascertained facts which
seem to point to the transmission of artificially produced diseases
cannot be considered as a proof; and as long as such proof is wanting
we have no right to make this supposition, unless compelled to do so by
the impossibility of suggesting a mode in which the transformation of
species can take place without its aid. (See Appendix IV, p. 310.)

It is obvious that the unconscious conviction that we need the aid of
acquired characters has hitherto securely maintained the assumed axiom
of the transmission of such features. It was believed that we could not
do without such an axiom in order to explain the transformation of
species; and this was believed not only by those who hold that the
direct action of external influences plays an important part in the
process, but also by those who hold that the operation of natural
selection is the main factor.

Individual variability forms the most important foundation of the
theory of natural selection: without it the latter could not exist, for
this alone can furnish the minute differences by the accumulation of
which new forms are said to arise in the course of generations. But how
can such hereditary individual characters exist if the changes wrought
by the action of external influences, during the life of an individual,
cannot be transmitted? We are clearly compelled to find some other
source of hereditary individual differences, or the theory of natural
selection would collapse, as it certainly would if hereditary
individual variations did not exist. If, on the other hand, acquired
differences are transmitted, this would prove that there must be
something wrong in the theory of the continuity of the germ-plasm, as
above described, and in the non-transmission of acquired characters
which results from this theory. But I believe that it is possible to
suggest that the origin of hereditary individual characters takes place
in a manner quite different from any which has been as yet brought
forward. To explain this origin is the task which I am about to
undertake in the following pages.

The origin of individual variability has been hitherto represented
somewhat as follows. The phenomena of heredity lead to the conclusion
that each organism is capable of producing germs, from which,
theoretically at least, exact copies of the parent may arise. In
reality this is never the case, because each organism possesses the
power of reacting on the different external influences with which it is
brought into contact, a power without which it could neither develope
nor exist. Each organism reacting in a different way must be to some
extent changed. Favourable nutrition makes such an organism strong and
large; unfavourable nutrition renders it small and weak, and what is
true of the whole organism may also be said of its parts. Now it is
obvious that even the children of the same mother meet with influences
different in kind and degree, from the very beginning of their
existence, so that they must necessarily become unlike, even if we
suppose them to have been derived from absolutely identical germs, with
precisely the same hereditary tendencies.

In this manner individual differences are believed to have been
introduced. But if acquired characters are not transmitted the whole
chain of argument collapses, for none of those changes which are caused
by the conditions of nutrition acting upon single parts of the whole
organism, including the results of training and of the use or disuse of
single organs,—none of these changes can furnish hereditary
differences, nor can they be transmitted to succeeding generations.
They are, as it were, only transient characters as far as the species
is concerned.

The children of accomplished pianists do not inherit the art of playing
the piano; they have to learn it in the same laborious manner as that
by which their parents acquired it; they do not inherit anything except
that which their parents also possessed when children, viz. manual
dexterity and a good ear. Furthermore, language is not transmitted to
our children, although it has been practised not only by ourselves but
by an almost endless line of ancestors. Only recently, facts have again
been worked up and brought together, which show that children of highly
civilized nations have no trace of a language when they have grown up
in a wild condition and in complete isolation[182]. The power of speech
is an acquired or transient character: it is not inherited, and cannot
be transmitted: it disappears with the organism which manifests it. Not
only do similar phenomena occur in the vegetable kingdom, but they
present themselves in an especially striking manner.

When Nägeli[183] introduced Alpine plants, taken from their natural
habitat, into the botanical garden at Munich, many of the species were
so greatly altered that they could hardly be recognized: for instance,
the small Alpine hawk-weeds became large and thickly branching, and
they blossomed freely. But if such plants, or even their descendants,
were removed to a poor gravelly soil the new characters entirely
disappeared, and the plants were re-transformed into the original
Alpine form. The re-transformation was always complete, even when the
species had been cultivated in rich garden soil for several generations.

Similar experiments with identical results were made twenty years ago
by Alexis Jordan[184], who chiefly made use of _Draba verna_ in his
researches. These experiments furnish very strong proofs, because they
were originally undertaken without the bias which may be given by a
theory. Jordan only intended to decide experimentally whether the
numerous forms of the plant, as it occurs wild in different habitats,
are mere varieties or true species. He found that the different forms
do not pass into one another, and are in all cases re-transformed after
they have been altered by cultivation in a soil different from that in
which they usually grow, and he therefore assumed that they were true
species. All these experiments therefore confirm the conclusion that
external influences may alter the individual, but that the changes
produced are not transmitted to the germs, and are never hereditary.

Nägeli indeed asserts that innate individual differences do not exist
in plants. The differences which we find, for instance, between two
beeches or oaks, are always, according to him, modifications produced
by the influence of varying local conditions. But it is obvious that
Nägeli goes too far in this respect, although it may be conceded that
innate individual differences in plants are much more difficult to
distinguish from those which are acquired, than in animals.

There is no doubt about the occurrence of innate and hereditary
individual characters in animals, and we may find an especially
interesting illustration in the case of man. The human eye can with
practice appreciate the most minute differences between individual men,
and especially differences of feature. Every one knows that
peculiarities of feature persist in certain families through a long
series of generations. I need hardly remind the reader of the broad
forehead of the Julii, the projecting chin of the Hapsburgs, or the
curved nose of the Bourbons. Hence every one can see that hereditary
individual characters do unquestionably exist in man. The same
conclusion may be affirmed with equal certainty for all our domestic
animals, and I do not see any reason why there should be any doubt
about its application to other animals and to plants.

But now the question arises,—How can we explain the presence of such
characters consistently with a belief in the continuity of the
germ-plasm, a theory which implies the rejection of the supposition
that acquired characters can become hereditary? How can the individuals
of any species come to possess various characters which are undoubtedly
hereditary, if all changes which are due to the influence of external
conditions are transient and disappear with the individual in which
they arose? Why is it that individuals are distinguished by innate
characters, as well as by those which I have previously called
transient, and how can deep-seated hereditary characters arise at all,
if they are not produced by the external influences to which the
individual is exposed?

In the first place it may be argued that external influences may not
only act on the mature individual, or during its development, but that
they may also act at a still earlier period upon the germ-cell from
which it arises. It may be imagined that such influences of different
kinds might produce corresponding minute alterations in the molecular
structure of the germ-plasm, and as the latter is, according to our
supposition, transmitted from one generation to another, it follows
that such changes would be hereditary.

Without altogether denying that such influences may directly modify the
germ-cells, I nevertheless believe that they have no share in the
production of hereditary _individual_ characters.

The germ-plasm or idioplasm of the germ-cell (if this latter term be
preferred) certainly possesses an exceedingly complex minute structure,
but it is nevertheless a substance of extreme stability, for it absorbs
nourishment and grows enormously without the least change in its
complex molecular structure. With Nägeli we may indeed safely affirm so
much, although we are unable to acquire any direct knowledge as to the
constitution of germ-plasm. When we know that many species have
persisted unchanged for thousands of years, we have before us the proof
that their germ-plasm has preserved exactly the same molecular
structure during the whole period. I may remind the reader that many of
the embalmed bodies of the sacred Egyptian animals must be four
thousand years old, and that the species are identical with those now
existing in the same locality. Now, since the quantity of germ-plasm
contained in a single germ-cell must be very minute, and since only a
very small fraction can remain unchanged when the germ-cell developes
into an organism, it follows that an enormous growth of this small
fraction must take place in every individual, for it must be remembered
that each individual produces thousands of germ-cells. It is therefore
not too much to say that, during a period of four thousand years, the
growth of the germ-plasm in the Egyptian ibis or crocodile must have
been quite stupendous. But in the animals and plants which inhabit the
Alps and the far north, we have instances of species which have
remained unchanged for a much longer period, viz. for the time which
has elapsed between the close of the glacial epoch and the present day.
In such organisms the growth of the germ-plasm must therefore have been
still greater.

If nevertheless the molecular structure of the germ-plasm has remained
precisely the same, this substance cannot be readily modifiable, and
there is very little chance of the smallest changes being produced in
its molecular structure, by the operation of those minute transient
variations in nutrition to which the germ-cells, together with every
other part of the organism, are exposed. The rate of growth of the
germ-plasm will certainly vary, but its structure is unlikely to be
affected for the above-mentioned reasons, and also because the
influences are mostly changeable, and occur sometimes in one and
sometimes in another direction.

Hereditary individual differences must therefore be derived from some
other source.

I believe that such a source is to be looked for in the form of
reproduction by which the great majority of existing organisms are
propagated: viz. in sexual, or, as Häckel calls it, amphigonic
reproduction.

It is well known that this process consists in the coalescence of two
distinct germ-cells, or perhaps only of their nuclei. These germ-cells
contain the germ-substance, the germ-plasm, and this again, owing to
its specific molecular structure, is the bearer of the hereditary
tendencies of the organism from which the germ-cell has been derived.
Thus in amphigonic reproduction two groups of hereditary tendencies are
as it were combined. I regard this combination as the cause of
hereditary individual characters, and I believe that the production of
such characters is the true significance of amphigonic reproduction.
The object of this process is to create those individual differences
which form the material out of which natural selection produces new
species.

At first sight this conclusion appears to be very startling and almost
incredible, because we are on the contrary inclined to believe that the
continued combination of existing differences, which is implied by the
very existence of amphigonic reproduction, cannot lead to their
intensification, but rather to their diminution and gradual
obliteration. Indeed the opinion has already been expressed that
deviations from the specific type are rapidly destroyed by the
operation of sexual reproduction. Such an opinion may be true with
regard to specific characters, because the deviations from a specific
type occur in such rare cases that they cannot hold their ground
against the large number of normal individuals. But the case is
different with those minute differences which are characteristic of
individuals, because every individual possesses them, although of a
different kind and degree. The extinction of such differences could
only take place if a few individuals constituted a whole species; but
the number of individuals which together represent a species is not
only very large but generally incalculable. Cross-breeding between all
individuals is impossible, and hence the obliteration of individual
differences is also impossible.

In order to explain the effects of sexual reproduction, we will first
of all consider what happens in monogonic or unisexual reproduction,
which actually occurs in parthenogenetic organisms. Let us imagine an
individual producing germ-cells, each of which may by itself develope
into a new individual. If we then suppose a species to be made up of
individuals which are absolutely identical, it follows that their
descendants must also remain identical through any number of
generations, if we neglect the transient non-transmissible
peculiarities caused by differences of food and other external
conditions.

Although the individuals of such a species might be actually different,
they would be potentially identical: in the mature state they might
differ, but they must have been identical in origin. The germs of all
of them must contain exactly the same hereditary tendencies, and if it
were possible for their development to take place under exactly the
same conditions, identical individuals would be produced.

Let us now assume that the individuals of such a species, reproducing
itself by the monogonic process and therefore without cross-breeding,
differ, not only in transient but also in hereditary characters. If
this were the case, each individual would produce descendants
possessing the same hereditary differences which were characteristic of
itself; and thus from each individual a series of generations would
emanate, the single individuals of which would be potentially identical
with each other and with their first ancestor. Hence the same
individual differences would be repeated again and again, in each
succeeding generation, and even if all the descendants lived to
reproduce themselves, there would be at last just as many groups of
potentially identical individuals as there were single individuals at
the beginning.

Similar cases actually occur in many species in which sexual
reproduction has been entirely replaced by the parthenogenetic method,
as in many species of _Cynips_ and in certain lower Crustacea. But all
these differ from our hypothetical case in one important respect; it is
always impossible for all the descendants to reach maturity and
reproduce themselves. The vast majority of the descendants generally
perish at an early stage, and only about as many remain to continue the
species as reached maturity in the preceding generation.

We have now to consider whether such a species can be subject to the
operation of natural selection. Let us take the case of an insect
living among green leaves, and possessing a green colour as a
protection against discovery by its enemies. We will assume that the
hereditary individual differences consist of various shades of green.
Let us further suppose that the sudden extinction of its food-plant
compelled this species to seek another plant with a somewhat different
shade of green. It is clear that such an insect would not be completely
adapted to the new environment. It would therefore be compelled,
metaphorically speaking, to endeavour to bring its colour into closer
harmony with that of the new food-plant, or else the increased chances
of detection given to its enemies would lead to its slow but certain
extinction.

It is obvious that such a species would be altogether unable to produce
the required adaptation, for _ex hypothesi_, its hereditary variations
remain the same, one generation after another. If therefore the
required shade of green was not previously present, as one of the
original individual differences, it could not be produced at any time.
If, however, we suppose that such a colour existed previously in
certain individuals, it follows that those with other shades of green
would be gradually exterminated, while the former would alone survive.
But this process would not be an adaptation in the sense used in the
theory of natural selection. It would indeed be a process of selection,
but it could form no more than the beginning of that process which we
call natural selection. If the latter could only bring existing
characters into prominence, it would not be worth much consideration,
for it could never produce a new species. A species never includes,
from the beginning, individuals which deviate from the specific type as
widely as the individuals of the most nearly allied species deviate
from it. And it would be still less possible to explain, on such a
principle, the origin of the whole organic world; for, if so, all
existing species would have been included as variations of the first
species. Natural selection must be able to do infinitely more than
this, if it is to be of any importance as a principle of development.
It must be able to accumulate minute existing differences in the
required direction, and thus to create new characters. In our example
it ought to be able, after preserving those individuals with a colour
nearest to the required shade, to lead their descendants onward through
successive stages towards a complete harmony of colour.

But such a result is quite unattainable with the asexual method of
reproduction: in other words, natural selection, in the true meaning of
the term, viz. a process which could produce new characters in the
manner above described, is an impossibility in a species propagated by
asexual reproduction.

If it could be shown that a purely parthenogenetic species had become
transformed into a new one, such an observation would prove the
existence of some force of transformation other than selective
processes, for the new species could not have been produced by these
latter. As already explained, the only selection which would be
possible for such a species, would lead to the survival of one group of
individuals and to the extinction of all others. Thus in our example
that group of individuals would alone survive, the ancestors of which
originally possessed the appropriate colour. But if one group alone
survived, it follows that all hereditary individual differences would
have disappeared from the species, for the members of such a single
group are identical with one another and with their original ancestors.
We thus reach the conclusion that monogonic reproduction can never
cause hereditary individual variability, but that, on the other hand,
it is very likely to lead to its entire suppression.

But the case is very different with sexual reproduction. When once
individual differences have begun to appear in a species propagated by
this process, uniformity among its individuals can never again be
reached. So far from this being the case, the differences must even be
increased in the course of generations, not indeed in intensity, but in
number, for new combinations of the individual characters will
continually arise.

Again, assuming the existence of a number of individuals which differ
from one another by a few hereditary individual characters, it follows
that no individual of the second generation can be identical with any
other. They must all differ, not only actually but also potentially,
for their differences exist at the very beginning of development, and
do not solely depend upon the accidental conditions under which they
live. Moreover, no one of the descendants can be identical with any of
the ancestors, for each of the former unites within itself the
hereditary tendencies of two parents, and its organism is therefore, as
it were, a compromise between two developmental tendencies. Similarly
in the third generation, the hereditary tendencies of two individuals
of the second generation enter into combination. But since the
germ-plasm of the latter is not simple, but composed of two
individually distinct kinds of germ-plasm, it follows that an
individual of the third generation is a compromise between four
different hereditary tendencies. In the fourth generation, eight; in
the fifth, sixteen; in the sixth, thirty-two different hereditary
tendencies must come together, and each of them will make itself more
or less felt in some part of the future organism. Thus by the sixth
generation a large number of varied combinations of ancestral
individual characters will appear, combinations which have never
existed before and which can never exist again.

We do not know the number of generations over which the specific
hereditary tendencies of the first generation can make themselves felt.
Many facts seem to indicate however that the number is large, and it is
at all events greater than six. When we remember that, in the tenth
generation, a single germ contains 1024 different germ-plasms, with
their inherent hereditary tendencies, it is quite clear that continued
sexual reproduction can never lead to the re-appearance of exactly the
same combination, but that new ones must always arise.

New combinations are all the more probable because the different
idioplasms composing the germ-plasm in the germ-cells of any individual
are present in different degrees of intensity at different times of its
life; in other words, the intensity of the component idioplasms is a
function of time. This conclusion follows from the fact that children
of the same parents are never exactly identical. In one child the
characters of the father may predominate, in another those of the
mother, in another again those of either grand-parent or
great-grand-parent.

We are thus led to the conclusion that even in a few sexually produced
generations a large number of well-marked individuals must arise: and
this would even be true of generations springing from our hypothetical
species, assumed to be without ancestors, and characterised by few
individual differences. But of course organisms which reproduce
themselves sexually are never without ancestors, and if these latter
were also propagated by the sexual method, it follows that each
generation of every sexual species is in the stage which we have
previously assumed for the tenth or some much later generation of the
hypothetical species. In other words, each individual contains a
maximum of hereditary tendencies and an infinite variety of possible
individual characters (see Appendix VI, p. 326).

In this manner we can explain the origin of hereditary individual
variability as it is known in man and the higher animals, and as it is
required for the theory which explains the transformation of species by
means of natural selection.

Before proceeding further, I must attempt to answer a question which
obviously suggests itself. For the sake of argument, I have assumed the
existence of a first generation, of which the individuals were already
characterised by individual differences. Can we find any explanation of
these latter, or are we compelled to take them for granted, without any
attempt to enquire into their origin? If we abandon this enquiry, we
can never achieve a complete solution of the problems of heredity and
variability. We have, it is true, shown that hereditary differences,
when they have once appeared, would, through sexual reproduction,
undergo development into the diverse forms which actually exist; but
this conclusion affords us no explanation of the source whence such
differences have been derived. If the external conditions acting
directly upon an organism can only produce transient (viz.
non-hereditary) differences in the latter, and if, on the other hand,
the external influences which act upon the germ-cell can only produce a
change in its molecular structure after operating over very long
periods, it seems that we have exhausted all the possible sources of
hereditary differences without reaching any satisfactory explanation.

I believe, however, that an explanation can be given. The origin of
hereditary individual variability cannot indeed be found in the higher
organisms—the Metazoa and Metaphyta; but it is to be sought for in the
lowest—the unicellular organisms. In these latter the distinction
between body-cell and germ-cell does not exist. Such organisms are
reproduced by division, and if therefore any one of them becomes
changed in the course of its life by some external influence, and thus
receives an individual character, the method of reproduction ensures
that the acquired peculiarity will be transmitted to its descendants.
If, for instance, a Protozoon, by constantly struggling against the
mechanical influence of currents in water, were to gain a somewhat
denser and more resistent protoplasm, or were to acquire the power of
adhering more strongly than the other individuals of its species, the
peculiarity in question would be directly continued on into its two
descendants, for the latter are at first nothing more than the two
halves of the former. It therefore follows that every modification
which appears in the course of its life, every individual character,
however it may have arisen, must necessarily be directly transmitted to
the two offspring of a unicellular organism.

The pianist, whom I have already used as an illustration, may by
practice develope the muscles of his fingers so as to ensure the
highest dexterity and power; but such an effect would be entirely
transient, for it depends upon a modification in local nutrition which
would be unable to cause any change in the molecular structure of the
germ-cells, and could not therefore produce any effect upon the
offspring. And even if we admit that some change might be caused in the
germ-cells, the chances would be infinity to nothing against the
production of the appropriate effect, viz. such a change as would lead
to the development in the child of the acquired characters of the
parent.

In the lowest unicellular organisms, however, the case is entirely
different. Here parent and offspring are still, in a certain sense, one
and the same thing: the child is a part, and usually half, of the
parent. If therefore the individuals of a unicellular species are acted
upon by any of the various external influences, it is inevitable that
hereditary individual differences will arise in them; and as a matter
of fact it is indisputable that changes are thus produced in these
organisms, and that the resulting characters are transmitted. It has
been directly observed that individual differences do occur in
unicellular organisms,—differences in size, colour, form, and the
number or arrangement of cilia. It must be admitted that we have not
hitherto paid sufficient attention to this point, and moreover our best
microscopes are only very rough means of observation when we come to
deal with such minute organisms. Nevertheless we cannot doubt that the
individuals of the same species are not absolutely identical.

We are thus driven to the conclusion that the ultimate origin of
hereditary individual differences lies in the direct action of external
influences upon the organism. Hereditary variability cannot however
arise in this way at every stage of organic development, as biologists
have hitherto been inclined to believe. It can only arise in the lowest
unicellular organisms; and when once individual difference had been
attained by these, it necessarily passed over into the higher organisms
when they first appeared. Sexual reproduction coming into existence at
the same time, the hereditary differences were increased and
multiplied, and arranged in ever-changing combinations.

Sexual reproduction can also increase the differences between
individuals, because constant cross-breeding must necessarily and
repeatedly lead to a combination of forces which tend in the same
direction, and which may determine the constitution of any part of the
body. If, for instance, the same part of the body is strongly developed
in both parents, the experience of breeders tells us that the part in
question is likely to be even more strongly developed in the offspring;
and that weakly developed parts will in the same manner tend to become
still weaker. Amphigonic reproduction therefore ensures that every
character which is subject to individual fluctuation must appear in
many individuals with a strengthened degree of development, in many
others with a development which is less than normal, while in a still
larger number of individuals the average development will be reached.
Such differences afford the material by means of which natural
selection is able to increase or weaken each character according to the
needs of the species. By the removal of the less well-adapted
individuals, natural selection increases the chance of beneficial
cross-breeding in the subsequent generations.

Every one must admit that, if a species came into existence having only
a small number of individual differences which appeared in the
different parts of different individuals, the number of differences
would increase with each sexually produced generation, until all the
parts in which the variations occurred had received a peculiar
character in all individuals.

Moreover sexual reproduction not only adds to the number of existing
differences, but it also brings them into new combinations, and this
latter consequence is as important as the former.

The former consequence can hardly make itself felt in any existing
species, because in them every part already possesses its peculiar
character in all individuals. The second consequence is, however, more
important, viz. the production of new combinations of individual
characters by sexual reproduction; for, as Darwin has already pointed
out, we must imagine that not only are single characters changed in the
process of breeding, but that probably several, and perhaps very many
characters, are simultaneously modified. No two species, however nearly
allied, differ from each other in but a single character. Even our
eyesight, which has by no means reached the highest pitch of
development, can always detect several, and often very many points of
difference; and if we possessed the powers necessary for making an
absolutely accurate comparison, we should probably find that everything
is different in two nearly allied species.

It is true that a great number of these differences depend upon
correlation, but others must depend upon simultaneous primary changes.

A large butterfly (_Kallima paralecta_), found in the East Indian
forests, has often been described in its position of rest as almost
exactly resembling a withered leaf; the resemblance in colour being
aided by the markings which imitate the venation of a leaf. These
markings are composed of two parts, the upper of which is on the
fore-wings, while the lower one is on the hind wings. The butterfly
when at rest must therefore keep the wings in such a position that the
two parts of each marking exactly correspond, for otherwise the
character would be valueless; and as a matter of fact the wings are
held in the appropriate position, although the butterfly is of course
unconscious of what it is doing. Hence a mechanism must exist in the
insect’s brain which compels it to assume this attitude, and it is
clear that the mechanism cannot have been developed before the peculiar
manner of holding the wings became advantageous to the butterfly, viz.
before the similarity to a leaf had made its first appearance.
Conversely, this latter resemblance could not develope before the
butterfly had gained the habit of holding its wings in the appropriate
position. Both characters must therefore have come into existence
simultaneously, and must have undergone increase side by side: the
marking progressing from an imperfect to a very close similarity, while
the position of the wings gradually approached the attitude which was
exactly appropriate. The development of certain minute structural
elements of the central nervous system, and the appropriate
distribution of colouring matter on the wings, must have taken place
simultaneously, and only those individuals have been selected to
continue the species which possessed the favourable variations in both
these directions.

It is, however, obvious that sexual reproduction will readily afford
such combinations of required characters, for by its means the most
diverse features are continually united in the same individual, and
this seems to me to be one of its most important results.

I do not know what meaning can be attributed to sexual reproduction
other than the creation of hereditary individual characters to form the
material upon which natural selection may work. Sexual reproduction is
so universal in all classes of multicellular organisms, and nature
deviates so rarely from it, that it must necessarily be of pre-eminent
importance. If it be true that new species are produced by processes of
selection, it follows that the development of the whole organic world
depends on these processes, and the part that amphigony has to play in
nature, by rendering selection possible among multicellular organisms,
is not only important, but of the very highest imaginable importance.

But when I maintain that the meaning of sexual reproduction is to
render possible the transformation of the higher organisms by means of
natural selection, such a statement is not equivalent to the assertion
that sexual reproduction originally came into existence in order to
achieve this end. The effects which are now produced by sexual
reproduction did not constitute the causes which led to its first
appearance. Sexual reproduction came into existence before it could
lead to hereditary individual variability. Its first appearance must
therefore have had some other cause; but the nature of this cause can
hardly be determined with any degree of certainty or precision from the
facts with which we are at present acquainted. The general solution of
the problem will, however, be found to lie in the conjugation of
unicellular organisms, which forms the precursor of true sexual
reproduction. The coalescence of two unicellular individuals which
represents the simplest and therefore probably the most primitive form
of conjugation, must have some directly beneficial effect upon the
species in which it occurs.

Various assumptions may be made as to the nature of these beneficial
effects, and it will be useful to consider in detail some of those
suggestions which have been brought forward. Eminent biologists, such
as Victor Hensen[185] and Edouard van Beneden[186], believe that
conjugation, and indeed sexual reproduction generally, must be
considered as ‘a rejuvenescence of life.’ Bütschli also accepts this
view, at any rate as regards conjugation. These authorities imagine
that the wonderful phenomena of life, of which the underlying cause is
still an unsolved problem, cannot be continued indefinitely by the
action of forces arising from within itself, that the clock-work would
be stopped after a longer or shorter time, that the reproduction of
purely asexual organisms would cease, just as the life of the
individual finally comes to an end, or as a spinning wheel comes to
rest in consequence of friction, and requires a renewed impetus if its
motion is to continue. In order that reproduction may continue without
interruption, these writers believe that a rejuvenescence of the living
substance is necessary, that the clock-work of reproduction must be
wound up afresh; and they recognize such a rejuvenescence in sexual
reproduction and in conjugation, or in other words in the fusion of two
cells, whether in the form of germ-cells or of two unicellular
organisms.

Edouard van Beneden expresses this idea in the following words:—‘Il
semble que la faculté que possèdent les cellules, de se multiplier par
division soit limitée: il arrive un moment où elles ne sont plus
capables de se diviser ultérieurement, à moins qu’elles ne subissent le
phénomène du rajeunissement par le fait de la fécondation. Chez les
animaux et les plantes les seules cellules capables d’être rajeunies
sont les œufs; les seules capables de rajeunir sont les spermatocytes.
Toutes les autres parties de l’individu sont vouées à la mort. La
fécondation est la condition de la continuité de la vie. Par elle le
générateur échappe à la mort’ (l. c., p. 405). Victor Hensen thinks it
possible that the germ and its products are prevented from dying by
means of normal fertilization: he says that the law which states that
every egg must be fertilized, was formulated before the discovery of
parthenogenesis and cannot now be maintained, but that we are
nevertheless compelled to assume that even the most completely
parthenogenetic species requires fertilization after many generations
(l. c., p. 236).

If the theory of rejuvenescence be thoroughly examined, it will be
found to be nothing more than the expression of the fact that sexual
reproduction persists without any ascertainable limit. From the fact of
its general occurrence, the conclusion is, however, drawn that asexual
reproduction could not persist indefinitely as the only mode of
reproduction in any species of animal. But proofs in support of this
opinion are wanting, and it is very probable that it would never have
been advanced if it had been possible to explain the general occurrence
of sexual reproduction in any other way,—if we had been able to ascribe
any other significance to this pre-eminently important process.

But quite apart from the fact that it is impossible to bring forward
any proofs, the theory of rejuvenescence seems to me to be
unsatisfactory in other ways. The whole conception of rejuvenescence,
although very ingenious, has something uncertain about it, and can
hardly be brought into accordance with the usual conception of life as
based upon physical and mechanical forces. How can any one imagine that
an Infusorian, which by continued division had lost its power of
reproduction, could regain this power by forming a new individual,
after fusion with another Infusorian, which had similarly become
incapable of division? Twice nothing cannot make one. If indeed we
could assume that each animal contained half the power necessary for
reproduction, then both together would certainly form an efficient
whole; but it is hardly possible to apply the term rejuvenescence to a
process which is simply an addition, such as would be attained under
other circumstances by mere growth; neglecting, for the present, that
factor which, in my opinion, is of the utmost importance in
conjugation,—the fusion of two hereditary tendencies. If rejuvenescence
possesses any significance at all, it must be this,—that by its means a
force, which did not previously exist in the conjugating individuals,
is called into activity. Such a force would, however, owe its existence
to latent energy stored up in each single animal during the period of
asexual reproduction, and such latent forces would necessarily be of
different natures, and of such a constitution that their union at the
moment of conjugation would give rise to the active force of
reproduction.

The process might perhaps be compared to the flight of two rockets,
which by the combustion of some explosive substance (such as
nitro-glycerine) stored up within themselves are impelled in such a
direction that they would meet at the end of their course, when all the
nitro-glycerine had been completely exhausted. The movement would then
come to an end, unless the explosive material could have been meanwhile
renewed. Now suppose that such a renewal were achieved by the formation
of nitric acid in one of the rockets and glycerine in the other, so
that when they came into contact nitro-glycerine would be formed afresh
equal in quantity and in distribution on both the rockets to that which
was originally present. In this way the movement would be renewed again
and again with the same velocity, and might continue for ever.

Rejuvenescence can be rendered intelligible in theory by some such
metaphor, but considerable difficulties are encountered in the rigid
application of the metaphor to the facts of the case. In the first
place, how is it possible that the motive force can be exhausted by
continual division, while one of its components is being formed afresh
in the same body and during the same time? When thoroughly examined the
loss of the power of division is seen to follow from the loss of the
powers of assimilation, nutrition, and growth. How is it possible that
such a power can be weakened and finally entirely lost while one of its
components is accumulated?

I believe that, instead of accepting such daring assumptions, it is
better to be satisfied with the simple conception of living matter
possessing as attributes the powers of unlimited assimilation and
capacity for reproduction. With such a theory the mere form of
reproduction, whether sexual or asexual, will have no influence upon
the duration of the capacity: for force and matter undergo simultaneous
increase, and are inseparably connected in this as in all other
instances. This theory does not, however, exclude the possible
occurrence of circumstances under which such an association is no
longer necessary.

I could only consent to adopt the hypothesis of rejuvenescence, if it
were rendered absolutely certain that reproduction by division could
never under any circumstances persist indefinitely. But this cannot be
proved with any greater certainty than the converse proposition, and
hence, as far as direct proof is concerned, the facts are equally
uncertain on both sides. The hypothesis of rejuvenescence is, however,
opposed by the fact of parthenogenesis; for if fertilization possesses
in any way the meaning of rejuvenescence, and depends upon the union of
two different forms of force and of matter, which thus produce the
power of reproduction, it follows that we cannot understand how it
happens that the same power of reproduction may be sometimes produced
from one form of matter, alone and unaided. Logically speaking,
parthenogenesis should be as impossible as that either nitric acid or
glycerine should separately produce the effect of nitro-glycerine. The
supposition has indeed been made that in the case of parthenogenesis,
one fertilization is sufficient for a whole series of generations, but
this supposition is not only incapable of proof, but it is contradicted
by the fact that certain eggs which may develope parthenogenetically
are also capable of fertilization. If, in this case, the power of
reproduction were sufficient for development, how is it that the egg is
also capable of fertilization; and if the power were insufficient, how
is it that the egg can develope parthenogenetically? And yet one and
the same egg (in the bee) can develope into a new individual, with or
without fertilization. We cannot escape this dilemma by making the
further assumption, which is also incapable of proof, that a smaller
amount of reproductive force is required for the development of a male
individual than for the development of a female. It is true that the
unfertilized eggs of the bee produce male individuals, while the
fertilized ones develope into females, but in certain other species the
converse association holds good, while in others, again, fertilization
bears no relation to the sex of the offspring.

Although the mere fact that parthenogenesis occurs at all is, in my
opinion, sufficient to disprove the theory of rejuvenescence, it is
well to remember that parthenogenesis is now the only method of
reproduction in many species (although we do not know the period of
time over which these conditions have extended), and is nevertheless
unattended by any perceptible decrease in fertility.

From all these considerations we may draw the conclusion that the
process of rejuvenescence, as described above, cannot be accepted
either as the existing or the original meaning of conjugation, and the
question naturally arises as to what other significance this latter
process can have possessed at its first beginning.

Rolph[187] has expressed the opinion that conjugation is a form of
nutrition, so that the two conjugating individuals, as it were, devour
each other. Cienkowsky[188] also regards conjugation as merely
‘accelerated’ assimilation. There is, however, not only an essential
difference but a direct contrast between the processes of conjugation
and nutrition. With regard to Cienkowsky’s view, Hensen[189] has well
said that ‘coalescence in itself cannot be an accelerated nutrition,
because even if we admit that both individuals are in want of
nourishment, it is impossible that the need can be supplied by this
process, unless one of them perishes and is really devoured.’ In order
that an animal may serve as the food of another, it must perish and
must be brought into a fluid form, and finally it must be assimilated.
In the case before us, however, two protoplasmic bodies are placed side
by side and coalesce, without either of them passing into the liquid
state. Two idioplasms unite, together with all the hereditary
tendencies contained in them; but although it is certain that nutrition
in the proper sense of the word cannot take place, because neither of
the animals receives an addition of liquid food by the coalescence, yet
the consequence of this process must be in one respect similar to that
of nutrition and growth:—the mass of the body and the quantity of the
forces contained in it undergo simultaneous increase. It is not
inconceivable that effects are by this means rendered possible, which
under the peculiar circumstances leading to conjugation, could not have
been otherwise produced.

I believe that this is at any rate the direction in which we shall have
to seek for the first meaning of conjugation and for its phyletic
origin. This first result and meaning of conjugation may be
provisionally expressed in the following formula:—conjugation
originally signified a strengthening of the organism in relation to
reproduction, which happened when from some external cause, such as
want of oxygen, warmth, or food, the growth of the individual to the
extent necessary for reproduction could not take place.

This explanation must not be regarded as equivalent to that afforded by
the theory of rejuvenescence; for the latter process is said to be
necessary for the continuance of reproduction, and ought therefore to
occur periodically quite independently of external circumstances; while
according to my theory, conjugation at first only occurred under
unfavourable conditions, and assisted the species to overcome such
difficulties.

But whatever the original meaning of conjugation may have been, it
seems to have become already subordinated in the higher Protozoa, as is
indicated by the changes in the course taken by this process. The
higher Protozoa when conjugating do not as a rule coalesce completely
and permanently[190] in the manner followed by the lower Protozoa, and
it seems to me possible, or even probable, that in the former the
process has already gained the full significance of sexual
reproduction, and is to be looked upon as a source of variability.

Whether this be so or not, I believe it is certain that sexual
reproduction could not have been entirely abandoned at any period since
the time when the Metazoa and Metaphyta first arose; for they derived
this form of reproduction from their unicellular ancestors.

We know that organs and characters which have persisted through a long
series of generations are transmitted with extreme tenacity, even when
they have ceased to be of any direct use to their immediate possessors.
The rudimentary organs in various animals, and not least in man, afford
very strong proofs of the soundness of this conclusion. Another example
has only recently been discovered in the sixth finger, which has been
shown to exist in the human embryo[191], a part which has only been
present in a rudimentary form ever since the origin of the
Amphibia[192]. Superfluous organs become rudimentary very slowly, and
enormous periods must elapse before they completely disappear, while
the older a character is, the more firmly it becomes rooted in the
organism. What I have above called the physical constitution of a
species is based upon these facts, and upon them depend the _tout
ensemble_ of inherited characters, which are adapted to one another and
woven together into a harmonious whole. It is this specific nature of
an organism which causes it to respond to external influences in a
manner different from that followed by any other organism, which
prevents it from changing in any way except along certain definite
lines of variation, although these may be very numerous. Furthermore
these facts ensure that characters cannot be taken at random from the
constitution of a species and others substituted for them. Such a
variation as a mammal wanting the firm axis of the backbone is an
impossibility, not only because the backbone is necessary as a support
to the body, but chiefly because this structure has been inherited from
times immemorial, and has become so impressed upon the mammalian
organization that any variation so great as to threaten its very
existence cannot now take place. The view here set forth of the origin
of hereditary variability by amphigonic reproduction, makes it clear
that an organism is in a state of continual oscillation only upon the
surface, so to speak, while the fundamental parts of its constitution,
which have been inherited from extremely remote periods, remain
unaffected.

Thus sexual reproduction itself did not cease after it had existed in
the form of conjugation through innumerable generations of the vast
numbers of species which have been included under the Protozoa; it did
not cease even when its original physiological significance had lost
its importance, either completely or in part. This process, however,
had come to possess a new significance which ensured its continuance,
in the enormous advantage conferred on a species by the power of
adapting itself to new conditions of life, a power which could only be
preserved by means of this method of reproduction. The formation of new
species which among the lower Protozoa could be achieved without
amphigony, could only be attained by means of this process in the
Metazoa and Metaphyta. It was only in this way that hereditary
individual differences could arise and persist. It was impossible for
amphigony to disappear, for each species in which it was preserved was
necessarily superior to those which had lost it, and must have replaced
them in the course of time; for the former alone could adapt itself to
the ever-changing conditions of life, and the longer sexual
reproduction endured, the more firmly was it necessarily impressed upon
the constitution of the species, and the more difficult its
disappearance became.

Sexual reproduction has nevertheless been lost in some cases, although
only at first in certain generations. Thus in the _Aphidae_ and in many
lower Crustacea, generations with parthenogenetic reproduction
alternate with others which reproduce themselves by the sexual method.
But in most cases it is clear that this partial loss of amphigony
conferred considerable advantages upon the species by giving increased
capabilities for the maintenance of existence. By means of partial
parthenogenesis a much more rapid increase in the number of individuals
could be attained in a given time, and this fact is of the highest
importance for the peculiar circumstances under which these species
exist. A species of Crustacean which inhabits rapidly drying pools, and
developes from winter-eggs which have remained dried up in the mud,
has, as a rule, only a very short time in which to secure the existence
of succeeding generations. The few sexual eggs which have escaped the
attacks of numerous enemies develope immediately after the first shower
of rain; the animals attain their full size in a few days and reproduce
themselves as virgin females. Their descendants are propagated in the
same manner, and thus in a short time almost incredible numbers of
individuals are formed, until finally the sexual eggs are again
produced. If now the pool dries up again, the existence of the colony
is secured, for the number of animals which produce sexual eggs is very
large, and the eggs themselves are of course far more numerous, so that
in spite of the destructive agencies to which they are subjected, there
will be every chance of the survival of a sufficient number to produce
a new generation at a later period. Here, therefore, sexual
reproduction has not been abandoned accidentally or from any internal
cause, but as an adaptation to certain definite necessities imposed
upon the organism by its surroundings.

It is, however, well known that there are certain instances in which
sexual reproduction has been altogether lost, and in which
parthenogenesis is the only form of propagation. In the animal kingdom,
such a condition chiefly occurs in species of which the closely-allied
forms exhibit the above-mentioned alternation between parthenogenesis
and amphigony, viz. in many _Cynipidae_ and _Aphidae_, and also in
certain freshwater and marine Crustacea. We may imagine that these
parthenogenetic species have arisen from forms with alternating methods
of reproduction, by the disappearance of the sexual phase.

In any particular case, it may be difficult to point out the motive by
which this change has been determined; but it is most probable that the
same conditions which originally caused the intercalation of a
parthenogenetic stage have been efficient in causing the gradual
disappearance of the sexual stage. If a species of Crustacean, with the
above-described alternating method of reproduction (heterogeny), were
killed off by its enemies on a larger scale than before, it is obvious
that the threatened extinction of the species could be checked by the
attainment of a correspondingly greater degree of fertility. Such
increased fertility might well be produced by pure parthenogenesis (see
Appendix V, p. 323), by means of which the number of egg-producing
individuals in all the previous sexual generations would be doubled.

In a certain sense, this would be the last and most extreme method by
means of which a species might secure continued existence, for it is a
method for which it would have to pay very dearly at a later period. If
my theory as to the causes of hereditary individual variability be
correct, it follows that all species with purely parthenogenetic
reproduction are sure to die out; not, indeed, because of any failure
in meeting the existing conditions of life, but because they are
incapable of transforming themselves into new species, or, in fact, of
adapting themselves to any new conditions. Such species can no longer
be subject to the process of natural selection, because, with the
disappearance of sexual reproduction, they have also lost the power of
combining and increasing those hereditary individual characters which
they possess.

All the facts with which we are acquainted confirm this conclusion, for
whole groups of purely parthenogenetic species or genera are never met
with, as would certainly be the case if parthenogenesis had been the
only method of reproduction through a successional series of species.
We always find it in isolated instances, and under conditions which
compel the conclusion that it has become predominant in the species in
question, and has not been transmitted from any preceding species.

There still remains a very different class of facts which, so far as we
can judge, are in accordance with my theory as to the significance of
sexual reproduction, and which may be quoted in its support. I refer to
the condition of functionless organs in species with parthenogenetic
reproduction.

Under the supposition that acquired characters cannot be
transmitted—and this forms the foundation of the views here set
forth—organs which are of no further use cannot become rudimentary in
the direct and simple manner in which it has been hitherto imagined
that degeneration takes place. It is true that an organ which does not
perform any function exhibits a marked decrease of strength and
perfection in the individual which possesses it, but such acquired
degradation is not transmitted to its descendants, and we must
therefore look for some other explanation of the firmly established
fact that organs do become rudimentary through a series of generations.
In seeking this explanation, we shall have to start from the
supposition that new forms are not only created by natural selection,
but are also preserved by its means. In order that any part of the body
of an individual of any species may be kept at the maximum degree of
development, it is necessary that all individuals possessing it in a
less perfect form must be prevented from propagation—they must succumb
in the struggle for existence. I will illustrate this by a special
instance. In species which, like the birds of prey[193], depend for
food upon the acuteness of their vision, all individuals with
relatively weak eyesight must be exterminated, because they will fail
in the competition for food. Such birds will perish before they have
reproduced themselves, and their imperfect vision is not further
transmitted. In this way the keen eyesight of birds of prey is kept up
to its maximum.

But as soon as an organ becomes useless, the continued selection of
individuals in which it is best developed must cease, and a process
which I have termed _panmixia_ takes place. When this process is in
operation, not only those individuals with the best-developed organs
have the chance of reproducing themselves, but also those individuals
in which the organs are less well-developed. Hence follows a mixture of
all possible degrees of perfection, which must in the course of time
result in the deterioration of the average development of the organ.
Thus a species which has retired into dark caverns must necessarily
come to gradually possess less developed powers of vision; for defects
in the structure of the eyes, which occur in consequence of individual
variability, are not eliminated by natural selection, but may be
transmitted and fixed in the descendants[194]. This result is all the
more likely to happen, inasmuch as other organs which are of importance
for the life of the species will gain what the functionless organ loses
in size and nutrition. As at each stage of retrogressive transformation
individual fluctuations always occur, a continued decline from the
original degree of development will inevitably, although very slowly,
take place, until the last remnant finally disappears. How
inconceivably slowly this process goes on is shown by the numerous
cases of rudimentary organs: by the above-mentioned embryonic sixth
finger of man, or by the hind limbs of whales buried beneath the
surface of the body, or by their embryonic tooth-germs. I believe that
the very slowness with which functionless organs gradually disappear,
agrees much better with my theory than with the one which has been
hitherto held. The result of the disuse of an organ is considerable,
even in the course of a single individual life, and if only a small
fraction of such a result were transmitted to the descendants, the
organ would be necessarily reduced to a minimum, in a hundred or at any
rate in a thousand generations. But how many millions of generations
may have elapsed since e. g. the teeth of the whalebone whales became
useless, and were replaced by whalebone! We do not know the actual
number of years, but we know that the whole material of the tertiary
rocks has been derived from the older strata, deposited in the sea,
elevated, and has been itself largely removed by denudation, since that
time.

Now if this theory as to the causes of deterioration in disused organs
be correct, it follows that rudimentary organs can only occur in
species with sexual reproduction, and that they cannot be formed in
species which are exclusively reproduced by the parthenogenetic method:
for, according to my theory, variability depends upon sexual
reproduction, while the deterioration of an organ when disused, no less
than its improvement when in use, depends upon variability. There are
therefore two reasons which lead us to expect that organs which are no
longer used will remain unreduced in species with asexual reproduction:
first, because only a very slight degree of hereditary variability can
be present, viz. such a degree as was transmitted from the time when
sexual reproduction was first abandoned by the ancestors; and,
secondly, because even these slight degrees of variability are not
combined, or, in other words, because panmixia cannot occur.

And the facts seem to point in the direction required by the theory,
for superfluous organs do not become rudimentary in parthenogenetic
species. For example, as far as my experience goes, the _receptaculum
seminis_ does not deteriorate, although it is, of course, altogether
functionless when parthenogenesis has become established. I do not
attach much importance to the fact that the Psychids and
Solenobias—(genera of Lepidoptera which Siebold and Leuckart have shown
to include species with parthenogenetic reproduction)—still retain the
complete female sexual apparatus, because colonies containing males
still occasionally occur in these species. Although the majority of
colonies are now purely female, the occasional appearance of males
points to the fact that the unisexuality of the majority cannot have
been of very long duration. The process of transformation of the
species from a bisexual into a unisexual form, only composed of
females, is obviously incomplete, and is still in process of
development. The case is similar with several species of _Cynipidae_,
which reproduce by the parthenogenetic method. In these cases the
occurrence of a very small proportion of males is the general rule, and
is not confined to single colonies. Thus Adler[195] counted 7 males and
664 females in the common _Cynips_ of the rose.

In some Ostracodes, on the other hand, the males appear to be entirely
wanting: at least, I have tried in vain for years to discover them in
any locality or at any time of the year[196].

_Cypris vidua_ and _Cypris reptans_ are such species. Now, although the
transformation of these formerly bisexual species into purely unisexual
female species appears to be complete[197], yet the females still
possess a large, pear-shaped _receptaculum seminis_, with its long
spirally twisted duct, which is surrounded by a thick glandular layer.
This is the more remarkable as the apparatus is very complicated in the
Ostracodes, and retrogressive changes could be therefore easily
detected. Furthermore among insects, in the genus _Chermes_ the
_receptaculum seminis_ of the females has also remained unreduced,
although the males appear to be entirely wanting, or at least have
never been found, in spite of the united efforts of several acute
observers[198]. The case is quite different in species which retain
both sexual and parthenogenetic reproduction. Thus, the summer females
of the _Aphidae_ have lost the _receptaculum seminis_; and in these
insects sexual reproduction has not ceased, but alternates regularly
with parthenogenetic reproduction.

Certainly this proof of the truth of my theory as to the significance
of sexual reproduction is far from settling the question: it only
renders the theory highly probable. At present it is impossible to do
more than this, because we do not yet possess a sufficient number of
facts, for many of them could not have been sought for until after the
theory had been suggested. We are here concerned with complicated
phenomena, into which we cannot acquire an immediate insight, but can
only attain it gradually.

But, nevertheless, I hope to have shown that the theory of natural
selection is by no means incompatible with the theory of ‘the
continuity of the germ-plasm;’ and, further, that if we accept this
latter theory, sexual reproduction appears in an entirely new light: it
has received a meaning, and has to a certain extent become intelligible.

The time in which men believed that science could be advanced by the
mere collection of facts has long passed away: we know that it is not
necessary to accumulate a vast number of miscellaneous facts, or to
make as it were a catalogue of them; but we know that it is necessary
to establish facts which, when grouped together in the light of a
theory, will enable us to acquire a certain degree of insight into some
natural phenomenon. In order to direct our attention to those new facts
which are of immediate importance, it is absolutely necessary to seek
the aid of some general theory for the arrangement and grouping of
those which we already possess. This has been my object in the present
paper.

But it may be perhaps objected that these phenomena are far too
complicated to be attacked at the present time, and that we ought to
wait quietly until the simpler phenomena have been resolved into their
components. It may be asked whether the trouble and labour involved in
the attempt to solve such questions as heredity or the transformation
of species are not likely to be wasted and useless.

It is true that we sometimes meet with such opinions, but I believe
that they are based upon a misunderstanding of the method which mankind
has always followed in the investigation of nature, and which must
therefore be founded upon the necessary relations existing between
mankind and nature.

Science has often been compared to an edifice which has been solidly
built by laying stone upon stone, until it has gradually risen to
greater height and perfection. This comparison holds good up to a
certain point, but it leads us to easily overlook the fact that this
metaphorical building does not at any point rest upon the ground, and
that, at least up to the present time, it has remained floating in the
air. Not a single branch of science, not even Physics itself, has
commenced building from below; all branches have begun to build at
greater or less heights in the air, and have then built downwards: and
even Physics has not yet reached the ground, for it is still very
uncertain as to the nature of matter and force. In no single group of
phenomena can we begin with the investigation of ultimate causes,
because at this very point our means of reasoning stop short. We cannot
begin with ultimate phenomena and gradually lead up to those which are
more complicated: we cannot proceed synthetically and deductively,
building up the phenomena from below; but we must as a rule proceed
analytically and inductively, proceeding from above downwards.

No one will dispute these statements, but they are often forgotten, as
is proved by the above-mentioned objection. If we were only permitted
to attack the more complicated phenomena after gaining a complete
insight into the simpler ones, then all scientists would be physicists
and chemists, and not until Physics and Chemistry were done with should
we be permitted to proceed to the investigation of organic nature.
Under these circumstances we ought not to possess now any scientific
theory of medicine; for the study of pathological physiology could not
be commenced until normal physiology was completely known and
understood. Yet how great a debt is owing by normal to pathological
physiology! This is an example which enforces the conclusion that it is
not only permissible, but in the highest degree advantageous, for the
different spheres of phenomena to be attacked simultaneously.

Furthermore, if we had been compelled to proceed from the simple to the
complex, what would have become of the Theory of Descent, the influence
of which has advanced our knowledge of Biology to an altogether
immeasurable extent?

But in this often repeated criticism that we are not yet ready to
attack such complicated phenomena as heredity, is hidden still another
fallacy, for it is implied that facts become less certain in proportion
to the complexity of their causes. But is it less certain that the egg
of an eagle developes into an eagle, or that the peculiarities of the
father and mother are transmitted to the child, than that a stone falls
to the ground when its support is taken away? Again, is it not possible
to draw a perfectly distinct and certain conclusion as to the relative
quantity of the material basis of heredity, present in the germ-cells
of either parent, from the fact that the father and mother possess an
equal or nearly equal share in heredity? But it is really unnecessary
to argue in this way: why should we do more than re-affirm that such a
method of procedure in scientific investigation is the only way by
which we can gradually penetrate the hidden depths of natural phenomena?

No! Biology is not obliged to wait until Physics and Chemistry are
completely finished; nor have we to wait for the investigation of the
phenomena of heredity until the physiology of the cell is complete.
Instead of comparing the progress of science to a building, I should
prefer to compare it to a mining operation, undertaken in order to open
up a freely branching lode. Such a lode must not be attacked from one
point alone, but from many points simultaneously. From some of these we
should quickly reach the deep-seated parts of the lode, from others we
should only reach its superficial parts; but from every point some
knowledge of the complex _tout ensemble_ of the lode would be gained.
And the more numerous the points of attack, the more complete would be
the knowledge acquired, for valuable insight will be obtained in every
place where the work is carried on with discretion and perseverance.

But discretion is indispensable for a fruitful result; or, leaving our
metaphor, facts must be connected together by theories, if science is
to advance. Just as theories are valueless without a firm basis of
facts, so the mere collection of facts, without relation and without
coherence, is utterly valueless. Science is impossible without
hypotheses and theories: they are the plummets with which we test the
depth of the ocean of unknown phenomena, and thus determine the future
course to be pursued on our voyage of discovery. They do not give us
absolute knowledge, but they afford us as much insight as it is
possible for us to gain at the present time. To go on investigating
without the guidance of theories, is like attempting to walk in a thick
mist without a track and without a compass. We should get somewhere
under these circumstances, but chance alone would determine whether we
should reach a stony desert of unintelligible facts or a system of
roads leading in some useful direction; and in most cases chance would
decide against us.

In this sense I trust that the sign-post or compass which I offer may
be accepted. Even though it should be its fate to be replaced by a
better one at a later period, it will have fulfilled its object if it
enables science to advance for even a short distance.

------------------------------------------------------------------------

                         Footnotes for Essay V.

Footnote 176:

  C. Nägeli, ‘Mechanisch-physiologische Theorie der Abstammungslehre.’
  München u. Leipzig, 1884.

Footnote 177:

  ‘Ueber die Berechtigung der Darwin’schen Theorie.’ Leipzig, 1868, p.
  27.

Footnote 178:

   l. c., Preface, p. vi.

Footnote 179:

  Since the above was written many other morphological peculiarities of
  plants have been rightly explained as adaptations. Compare, for
  instance, the investigations of Stahl on the means by which plants
  protect themselves against the attacks of snails and slugs (Jena,
  1888).—A. W., 1888.

Footnote 180:

  l. c., pp. 117, 286.

Footnote 181:

  Compare the second and fourth of the preceding Essays, ‘On Heredity’
  and ‘The Continuity of the Germ-plasm as the Foundation of a Theory
  of Heredity.’

Footnote 182:

  Compare Rauber, ‘Homo sapiens ferus oder die Zustände der
  Verwilderten.’ Leipzig, 1885.

Footnote 183:

  ‘Sitzungsberichte der baierischen Akademie der Wissenschaften,’ vom
  18 Nov. 1865. Compare also his ‘Mechanisch-physiologische Theorie der
  Abstammungslehre,’ p. 102, etc.

Footnote 184:

  Jordan, ‘Remarques sur le fait de l’existence en société des espèces
  végétales affines.’ Lyon, 1873.

Footnote 185:

  S. Hermann’s ‘Handbuch der Physiologie,’ Theil II; ‘Physiologie der
  Zeugung,’ by V. Hensen.

Footnote 186:

  E. van Beneden, ‘Recherches sur la maturation de l’œuf, la
  fécondation et la division cellulaire.’ Gand u. Leipzig, 1883, pp.
  404 et seq.

Footnote 187:

  Rolph, ‘Biologische Probleme.’ Leipzig, 1882.

Footnote 188:

  Cienkowsky, ‘Arch. f. mikr. Anat.,’ ix. p. 47. 1873.

Footnote 189:

  Hensen, ‘Physiologie der Zeugung,’ p. 139.

Footnote 190:

  Coalescence takes place in the so-called bud-like conjugation of
  _Vorticellidae_ and _Trichodinidae_, etc.

Footnote 191:

  Compare (1) Bardeleben, ‘Zur Entwicklung der Fusswurzel,’
  Sitzungsber. d. Jen. Gesellschaft, Jahrg. 1885, Feb. 6; also
  ‘Verhandl. d. Naturforscherversammlung zu Strassburg,’ 1885, p. 203;
  (2) G. Baur, ‘Zur Morphologie des Carpus und Tarsus der
  Wirbelthiere,’ Zool. Anzeiger, 1885, pp. 326, 486.

Footnote 192:

  In frogs the sixth toe exists in the hind legs as a rudimentary
  prehallux. Compare Born, Morpholog. Jahrbuch, Bd. I, 1876.

Footnote 193:

   I here make use of the same illustration which I employed in my
  first attempt to explain the effects of _panmixia_. Compare the
  second Essay ‘On Heredity.’

Footnote 194:

  [E. Ray Lankester has suggested (Encycl. Britann., art. ‘Zoology,’
  pp. 818, 819) that the blindness of cave-dwelling and deep-sea
  animals is also due to the fact that ‘those individuals with perfect
  eyes would follow the glimmer of light and eventually escape to the
  outer air or the shallower depths, leaving behind those with
  imperfect eyes to breed in the dark place. A natural selection would
  thus be effected.’ Such a sifting process would certainly greatly
  quicken the rate of degeneration due to panmixia alone.—E. B. P.]

Footnote 195:

  Adler, ‘Zeitschrift f. wiss. Zool.,’ Bd. XXXV, 1881.

Footnote 196:

  Compare my paper, ‘Parthenogenese bei den Ostracoden,’ in ‘Zool.
  Anzeiger,’ 1880, p. 82. Purely negative evidence, unless on an
  immense scale, is quite rightly considered to be of no great value in
  most cases. But the condition of these animals renders the
  accumulation of such evidence unusually easy, because the presence of
  males in a colony of Ostracodes can be proved by a very simple
  indirect test. Thus if a colony contains any males the _receptacula
  seminis_ of all mature females are filled with spermatozoa, and on
  the other hand we may be quite sure that males are absent, if after
  the examination of many mature females, no spermatozoa can be found
  in any of their _receptacula_.

Footnote 197:

  We cannot, however, be absolutely certain of this, for it is
  conceivable that males may still occur in colonies other than those
  examined.

Footnote 198:

  It has now been shown by Blochmann that males appear for a very short
  time towards the close of summer, as in the case of _Phylloxera_.—A.
  W., 1888.

------------------------------------------------------------------------


                              APPENDICES.

        Appendix I. Further considerations which oppose Nägeli’s
     explanation of transformation as due to internal causes[199].

When I describe Nägeli’s theory of transformation as due to active
causes lying within the organism, as a phyletic force of
transformation, I do not mean to imply that it is one of those
mysterious principles which, according to some writers, constitute the
unconscious cause which directs the transformation of species. Nägeli’s
idioplasm, which changes from within itself, is conceived as a
thoroughly scientific, mechanically operating principle. This cause is
undoubtedly capable of theoretical conception: the only question is
whether it has any real existence. According to Nägeli, the growing
organic substance, the idioplasm, not only represents a _perpetuum
mobile_ rendered possible as long as its substance continually receives
from without the matter and force which are necessary for continuous
growth, but it also represents a _perpetuum variabile_ due to the
action of internal causes[200]. But this is just the doubtful point,
viz., whether the structure of the idioplasm itself compels it to
change gradually during the course of its growth, or whether it is not
rather the external conditions which compel the ever slightly varying
idioplasm to change in a certain direction by the summation of small
differences. It has been shown above that we do not gain anything by
adopting Nägeli’s theory, because the main problem which organic nature
offers for our solution, viz. adaptation, remains unsolved. Hence this
theory does not explain the phenomena of nature, and I believe that
there are also certain facts which are directly antagonistic to it.

If the idioplasm really possessed the power of spontaneous variability
ascribed to it by Nägeli; if, as a result of its own growth, it were
compelled to undergo gradual changes, and thus to produce new species,
we should expect that the duration of species, genera, orders, &c.
would be of approximately equal length respectively, at least in forms
of equal structural complexity. The time required by the idioplasm to
undergo such changes as would constitute transformation into a new
species ought to be always the same at equal heights in the scale of
organization, that is, with equal complexity in the molecular structure
of the idioplasm. It appears to me to be a necessary consequence of
Nägeli’s theory that the causes of transformation lie solely in this
molecular structure of the idioplasm. If nothing more than a certain
amount of growth, and consequently a certain period of time during
which the organism reproduces itself with a certain intensity, is
required to produce a change in the idioplasm, then we must conclude
that the alteration in the latter must take place when this certain
amount of growth has been reached, or after this certain period has
elapsed. In other words, the time during which a species exists—from
its origin as a modification of some older species, until its own
transformation into a new one—must be the same in species with the same
degree of organization. But the facts are very far from supporting this
consequence of Nägeli’s theory. The duration of species is excessively
variable: many arise and perish within the limits of a single
geological formation, while others may be restricted to a very small
part of a formation; others again may last through several formations.
It must be admitted that we cannot estimate the exact position of
extinct species in the scale of organization, and the differences may
therefore depend upon differences of organization: or they may be
explained by the supposition that certain species may have become
incapable of transformation, and might, under favourable conditions,
continue to exist for an indefinite period. But this reply would
introduce a new hypothesis in direct antagonism to Nägeli’s theory,
which assumes that the variability of idioplasm takes place as the
consequence of mere growth, and necessarily depends upon molecular
structure. Nägeli himself asserts that the essential substance
(idioplasm) of the descendants of the earliest forms of life is in a
state of perpetual change, which would continue even if the series of
successive generations were indefinitely prolonged[201]. Hence there
can be no rest in the process of change which the idioplasm must
undergo; and this is as true of each single species as it is of the
organic world taken as a whole. We could, perhaps, find shelter in the
insufficiency of our geological knowledge, but the number of
ascertained facts is too great for this to be possible. Thus it is well
known that the genus _Nautilus_ has lasted from Silurian times, through
all the three geological periods, up to the present day: while all its
Silurian allies (_Orthoceras_, _Gomphoceras_, _Goniatites_, &c.) became
extinct at a comparatively early period.

A keen and clever controversialist might still bring forward many
objections against such an argument. I do not therefore place too much
dependence upon the geological facts by themselves, as a disproof of
the self-variability of Nägeli’s idioplasm; for it must be admitted
that the facts are not sufficiently complete for this purpose. For
instance, in the case of _Nautilus_ it might be argued that we do not
know anything about the fossil Cephalopods of pre-Silurian times, and
that it is therefore possible that the above-mentioned allies of
_Nautilus_ may have existed previously for as long a period as that
through which _Nautilus_ has lived in post-Silurian time. However this
may be, it will be at least conceded that the geological facts do not
lend any support to Nägeli’s theory, for we can see no trace of even an
approximately regular succession of forms.


         Appendix II. Nägeli’s explanation of adaptation[202].

In order to explain adaptation Nägeli assumes that, under certain
circumstances, external influences may cause slight permanent changes
in the idioplasm. If then such influences act continually in the same
direction during long periods of time, the changes in the idioplasm may
increase to a perceptible amount, i. e. to a degree which makes itself
felt in visible external characters[203]. But such changes alone could
not be considered as adaptations, for the essential character of an
adaptation is that it must be a purposeful change. Nägeli, however,
brings forward the fact that external stimuli often produce their chief
effects at that very part of the organism to which the stimuli
themselves were applied. ‘If the results are detrimental, the organism
attempts to defend itself against the stimulus: a confluence of
nutrient fluid takes place towards the part upon which the stimulus has
acted, and new tissues are formed which restore the integrity of the
organism by replacing the lost structures as far as possible. Thus in
plants the healthy tissues begin to grow actively around the seat of an
injury, tending to close it up, and to afford protection by
impenetrable layers of cork.’ Purposeful reactions of this kind are
certainly common in the organic world, occurring in animals as well as
in plants. Thus in the human body an injury causes a rapid growth of
the surrounding tissues, which leads to the closing-up of the wound;
while in the Salamander even the amputated leg or tail is replaced by
growth. An extreme example of these purposeful reactions is afforded by
the tree-frog (_Hyla_), which is of a light-green colour when seated
upon a light-green leaf, but becomes dark brown when transferred to
dark surroundings. Hence this animal adapts itself to the colour of its
environment, and thus gains protection from its enemies.

Admitting this capability on the part of organisms to react under
certain stimuli in a purposeful manner, the question remains whether
such a power is a primitive original quality belonging to the essential
nature of each organism. The power of changing the colour of the skin
in correspondence with that of the surroundings is not very common in
the animal kingdom. In the frog this power depends upon a highly
complex reflex mechanism. Certain chromatophores in the skin are
connected with nerves[204] which pass to the brain and are there
brought into relation, by means of nerve-cells, with the nervous
centres of the organ of vision. The relation is of such a kind that
strong light falling upon the retina constitutes a stimulus for the
production of an impulse, which is conducted, along the previously
mentioned motor nerves, from the brain to the chromatophores, thus
determining the contraction of these latter and the consequent
appearance of a light-coloured skin. When the strong stimulus (of
light) ceases, the chromatophores expand again, and the skin becomes
dark. That the chromatophores do not themselves react upon the direct
stimulus of light was proved by Lister[205], who showed that blind
frogs do not possess the power of altering their colour in
correspondence with that of their environment. It is quite obvious that
in this case we are not dealing with a primary, but with a secondarily
produced character; and it has yet to be proved that all the purposeful
reactions mentioned by Nägeli are not similarly secondary characters or
adaptations, and thus very far from being primitive qualities of the
organic substance of the forms in which they occur.

I do not by any means doubt that some of the reactions witnessed in
organisms do not depend upon adaptation, but such reactions are not
usually purposeful. Curiously enough, Nägeli mentions the formation of
galls in plants among his instances of purposeful reactions under
external stimuli. I think, however, that it can hardly be maintained
that the galls are of any use to the plant: on the contrary, they may
even be very injurious to it. The gall is only useful to the insect
which it protects and supplies with food. The recent and most excellent
investigations of Adler[206] and of Beyerinck[207] have shown that the
puncture made by the _Cynips_ in depositing its eggs is not the
stimulus which produces the gall, as was formerly believed to be the
case, but that such a stimulus is provided by the larva which developes
from the egg. The presence of this small, actively moving, foreign body
stimulates the tissue of the plant in a definite manner, always
producing a result which is advantageous to the larva and not to the
plant. It would be to the advantage of the latter if it killed the
intruding larva, either enclosing it by woody tissue devoid of
nourishment, or poisoning it by some acrid secretion, or simply
crushing it by the active growth of the surrounding tissues. But
nothing of the kind occurs: in fact an active growth of cells (forming
the so-called ‘Blastem’ of Beyerinck) takes place around the embryo,
while it is still enclosed in the egg-capsule; but the growth is not
such as to crush the embryo, which remains free in the cavity, the
so-called larval chamber, which is formed around it. It would be out of
place to discuss here the question as to how we can conceive that the
plant is thus compelled to produce a growth which is at any rate
indifferent and may be injurious to it; and which, moreover, is exactly
adapted to the needs of its insect-enemy. But it is at all events
obvious that this cannot be an example of a self-protecting reaction
under a stimulus, and that therefore an organism does not always
respond to external stimuli in a manner useful to itself.

But even if we could accept the suggestion that the purposeful reaction
of an organism under stimulation is a primary and not a secondarily
produced character, such a principle would by no means suffice for the
explanation of existing adaptations. Nägeli attempts to explain certain
selected cases of adaptation as the direct results of external stimuli.
He looks upon the thick hairy coat of mammals in arctic regions, and
the winter covering of animals in temperate regions, as a direct
reaction of the skin under the influence of cold. He considers that the
horns, claws, and tusks of animals have arisen directly as reactions
under stimuli applied to certain parts of the surface of the body in
attack and defence[208]. This interpretation is similar to that offered
by Lamarck at the beginning of this century. At first sight such a
suggestion appears to be plausible, for the acquisition of a thick
hairy covering by the mammals of temperate regions is actually
contemporaneous with the cold season of the year. But the question
arises as to whether the production of a larger number of hairs at the
beginning of winter is not merely another instance of a secondary
character, like the assumption of a green colour by the tree-frog under
the stimulus exerted by strong light.

In the case of the hairy coat it is only necessary to produce a larger
number of structures such as had existed previously; but how can it
have been possible for the petals of flowers, with their peculiar and
complex forms, to have been developed from stamens as a direct result
of the insects which visit them in order to obtain pollen and nectar?
How could the creeping of these insects and the small punctures made by
them constitute stimuli for the production of an increased rate of
growth? And how is it possible in any way to explain, by mere increase
in growth, the origin of a structure in which each part has its own
distinct meaning and plays a peculiar part in attracting insects and in
the process of cross-fertilization effected by them? Even if the
manifold peculiarities of form could be explained in this way, how can
such an explanation possibly hold for the colours of flowers? How could
the white colour of flowers which open at night be explained as the
direct result of the creeping of insects? How can the suggestion of
such a cause offer any interpretation of the fact that flowers which
open by day are tinted with various colours, or of the fact that there
is often a bright or highly coloured spot which shows the way to the
hidden nectary?

There are, moreover, a large number of very striking adaptations in
form and colour, for which no stimulus acting directly upon the
organism can be found. Can we imagine that the green caterpillar[209],
plant-bug, or grasshopper, sitting among green surroundings, is thus
exposed to a stimulus which directly produces the green colour in the
skin? Can the walking-stick insect, which resembles a brown twig, be
subject to a transforming stimulus by sitting on such branches or by
looking at them? Or again, if we consider the phenomena of mimicry, how
can one species of butterfly, by flying about with another species,
exercise upon the latter such an influence as to render it similar to
the first in appearance? In many cases of mimicry, the mimicked and the
mimicking species do not even live in the same place, as we see in the
moths, flies, and beetles which resemble in appearance the much-dreaded
wasps.

The interpretation of adaptation is the weak part of Nägeli’s theory,
and it is somewhat remarkable that so acute a thinker should not have
perceived this himself. One very nearly gains the impression that
Nägeli does not wish to understand the theory of natural selection. He
says, for instance, in speaking of the mutual adaptation observable
between the proboscis, the so-called ‘tongue’ of butterflies, and
flowers with tubular corolla[210]:—‘Among the most remarkable and
commonest adaptations observable in the forms of flowers, are the
corollas with long tubes considered in relation to the long “tongues”
of insects, which suck the nectar from the bottom of the long narrow
tubes, and at the same time effect the cross-fertilization of the
plant. Both these arrangements have been gradually developed to their
present degree of complexity—the long-tubed corollas from those without
tubes, and from those with short ones, the long “tongues” from short
ones. Undoubtedly both have been developed at the same rate so that the
length of both sets of structures has always remained the same.’

No objection can be raised against these statements, but Nägeli goes on
to say:—‘But how can such a process of development be explained by the
theory of natural selection, for at each stage in the process the
adaptation was invariably complete. The tube of the corolla and the
“tongue” must have reached, for instance, at a certain time, a length
of 5 or 10 mm. If now the tube of the corolla became longer in some
plants, such an alteration would have been disadvantageous because the
insects would be no longer able to obtain food from them, and would
therefore visit flowers with shorter tubes. Hence, according to the
theory of natural selection, the longer tubes ought to have
disappeared. If on the other hand the “tongue” became longer in some
insects, such a change would be superfluous and should have been given
up, according to the same theory, as unnecessary structural waste. The
simultaneous change in the two structures must, according to the theory
of natural selection, be due to the same principle as that by which
Münchhausen pulled himself out of a bog by means of his own pig-tail.’

But, according to the theory of natural selection, the case appears in
a very different light from that in which it is put by Nägeli. The
flower and the insect do not compete for the greater length of their
respective organs: all through the gradual process, the flower is the
first to lengthen its corolla and the butterfly follows. Their relation
is not like that between a certain species of animal and another which
serves as its prey, where each strives to be the quicker, so that the
speed of both is increased to the greatest possible extent in the
course of generations. Nor do they stand in the same relation as that
obtaining between an insectivorous bird and a certain species of
butterfly which forms its principal food; in such a case two totally
different characters may be continually increased up to their highest
point, e.g. in the butterfly similarity to the dead and fallen leaves
among which it seeks protection when pursued, in the bird keenness of
sight. As long as the latter quality is still capable of increase, so
long will it still be advantageous to any individual butterfly to
resemble the leaf a little more completely than other individuals of
the same species; for it will thus be capable of escaping those birds
which possess a rather keener sight than others. On the other hand, a
bird with rather keener sight will have the greatest chance of catching
the better protected butterflies. It is only in this way that we can
explain the constant production of such extraordinary similarities
between insects and leaves or other parts of plants. At every stage of
growth both the insect and its pursuer are completely adapted to each
other; i.e. they are so far protected and so far successful
respectively, as is necessary to prevent that gradual decrease in the
average number of individuals which would lead to the extermination of
the species[211]. But the fact that there is complete adaptation at
each stage does not prevent the two species from increasing those
qualities of protection and of pursuit upon which they respectively
depend. So far from this being the case, they would be necessarily
compelled to gradually increase these qualities so long as the physical
possibility of improvement remained on both sides. As long as some
birds possessed a rather keener sight than those which previously
existed, so long would those butterflies possess an advantage in which
the resemblance to leaf-veining was more distinct than in others. But
from the moment at which the maximum keenness of eyesight attainable
had been reached, at which therefore all butterflies resembled leaves
so completely that even the birds with the keenest eyesight might fail
to detect them when at rest,—from this very point any further
improvement in the similarity to leaves would cease, because the
advantage to be gained from any such improvement would cease at the
same time.

Such reciprocal intensification of adaptive characters appears to me to
have been one of the most important factors in the transformation of
species: it must have persisted through long series of species during
phylogeny: it must have affected the most diverse parts and characters
in the most diverse groups of organisms.

In certain large butterflies of the Indian and African forests—_Kallima
paralecta_, _K. inachis_, and _K. albofasciata_—it has been frequently
pointed out that the deceptive resemblance to a leaf is so striking
that an observer who has received no hint upon the subject believes
that he sees a leaf, even when he is looking at the butterfly very
closely. The similarity is nevertheless incomplete; for out of sixteen
specimens in the collections at Amsterdam and Leyden, I could not find
a single one which had more than two lateral veins on one side of the
mid-rib of the supposed leaf, or more than three upon the other side;
while about six or seven veins should have been present on each side.
But from two to three lateral veins are amply sufficient to produce a
high degree of resemblance; in fact so much so that it is a matter for
wonder as to how it has been possible for such a relatively perfect
copy to have been produced; or how the sight of birds can have become
so highly developed that while flying rapidly they could perceive the
vein-like markings; or to state the case more accurately, that they
could detect those individuals with a less number of veins than others.
It is possible that the process of increase in resemblance is still
proceeding in the species of the genus _Kallima_; at all events, I was
struck by the rather strong individual differences in the markings of
the supposed leaf.

On the other hand, the cause of the increase in length of the tubular
corolla and of the butterfly’s ‘tongue,’ lies neither in the flower nor
in the butterfly, but it is to be found in those other insects which
visit the flower and steal its honey without being of any assistance in
cross-fertilization. It may be stated shortly, that non-tubular
corollas, with the honey freely exposed—for it must be assumed the
ancestral form was of this kind—gradually developed into corollas with
the honey deeply concealed. The whole process was presumably first
started by the flower, for the gradual withdrawal of the honey to
greater depths conferred the advantage of protection from rain (Hermann
Müller), while larger quantities of honey could be stored up, and this
would also increase the number of insects visiting the flower and
render their visits more certain. As soon as this withdrawal occurred,
the mouth-parts of insects began to be subjected to a selective process
whereby these organs in some of them were lengthened at the same rate
as that at which the honey was withdrawn. When once the process had
begun, its continuance was ensured, for as soon as flower-frequenting
insects were divided into two groups with short and with long
mouth-parts respectively, a further increase in the length of the
corolla-tube necessarily took place in all those flowers which were
especially benefited by the assured visits of a relatively small number
of species of insects, viz., those flowers in which cross-fertilization
was more certainly performed in this way than by the uncertain visits
of a great variety of species. This would imply that a still further
increase in length would take place, for it is obvious that the
cross-fertilization of any flower would be more certainly performed by
an insect when the number of species of plants visited by it became
less; and hence the cross-fertilization would be rendered most certain
when the insect became completely adapted—in size, form, character of
its surface, and the manner in which it obtained the honey—to the
peculiarities of the flower. Those insects which obtain honey from a
great variety of flowers are sure to waste a great part of the pollen
by carrying it to the flowers of many different species, while insects
which can only obtain honey from a few species of plants must
necessarily visit many flowers of the same species one after the other,
and they would therefore more generally distribute the pollen in an
effective manner.

Hence the tube of the corolla, and the ‘tongue’ of the butterfly which
brings about fertilization, would have continued to increase in length
as long as it remained advantageous for the flower to exclude other
less useful visitors, and as long as it was advantageous for the
butterfly to secure the sole possession of the flower. Hence there is
no competition between the flower and the butterfly which fertilizes
it, but between these two on the one side, and the other would-be
visitors of the flower on the other. Further details as to the
advantages which the flower gains by excluding all other visitors, and
the butterfly by being the only visitor of the flower, and also as to
the manifold and elaborate mutual adaptations between insects and
flowers, and as to the advantages and disadvantages which follow from
the concealment of the honey—will be found in Hermann Müller’s[212]
work on the fertilization of flowers, in which all these subjects are
minutely discussed, and are clearly explained in a most admirable
manner.


               Appendix III. Adaptations in Plants[213].

It is well known that Christian Conrad Sprengel was the first to
recognise that the forms and colours of flowers are not due to chance,
that they are not the mere sport of nature, and that they are not made
for the enjoyment of man, but that their purpose is to attract insects
for the performance of cross-fertilization. It is also well known that
this discovery—which was made at the end of the last century, and which
caused much excitement at that time—was completely forgotten, and was
brought to light again by Charles Darwin when attacking the same
problem.

In his work entitled ‘The Solution of Nature’s Secret in the Structure
and Fertilization of Flowers’ (‘Das entdeckte Geheimniss der Natur im
Bau und der Befruchtung der Blumen’), published at Berlin, in 1793,
Sprengel showed, in several hundred cases, that the peculiarities in
the structure and colours of flowers were calculated to attract
insects, and to ensure the fertilization of the flowers by their
instrumentality. But it was due to his successor in this line of
investigation that the whole significance of the cross-fertilization
effected by insects was made clear. Darwin[214] showed that in many
cases, although not in all, the intention of nature was to avoid
self-fertilization, and he showed that stronger and more numerous
descendants are produced after cross-fertilization.

After Darwin, several investigators, such as Kerner, Delpino and
Hildebrand, have paid further attention to the subject, but it has
been especially studied in a most thorough manner by Hermann
Müller[215]. He looked at the subject from more than one point of
view, and showed by direct observation the species of insects which
effect cross-fertilization in various species of our native flowers:
he also studied the structure of insects in relation to that of
flowers, and attempted to establish the mutual adaptations which
exist between them. In this way he succeeded in throwing much light
upon the process of transformation in many species of flowers, and in
proving that certain insects, although unconsciously, are, as it
were, breeders of certain forms of flowers. He not only distinguished
the disagreeably smelling, generally inconspicuous flowers
(‘Ekelblumen’) produced by Diptera which live on putrid substances,
and the flowers which are produced by butterflies; but he also
distinguished the flowers bred by saw-flies, by Fossoria, and by
bees. He even believes that in certain cases (_Viola calcarata_) he
can prove that a flower which owed its original form to being bred by
bees, was afterwards adapted to cross-fertilization by butterflies,
when it had migrated into an Alpine region where the latter insects
are far more abundant than the former.

Although there must of course be much that is hypothetical in the
interpretations of the different parts of flowers offered by Hermann
Müller, the majority of these explanations are certainly correct, and
it is of the greatest interest to be able to recognise the adaptive
character of details, even when apparently unimportant, in the
structure and colours of flowers.

Sachs has offered a very convincing explanation as to the meaning of
leaf-veining, and of its significance in relation to the functions of
leaves[216]. He shows that the venation of a leaf is in every case
exactly adapted for the fulfilment of its purpose. It has, in the first
place, to conduct the nutrient fluid in both directions, and in the
second place to support the thin layers of assimilating chlorophyll
cells, and to stretch them out so as to expose as large a surface as
possible to the light; lastly, it has to toughen the leaf as a
protection against being torn. He shows in a very convincing manner
that the whole diversity of leaf venation can be understood from these
three principles. Here, again, we meet with purposeful arrangements in
a class of structures in which it was formerly thought that there was
only a chaos of accidental forms, or, as it were, the mere sport of
nature with form.


 Appendix IV. On the supposed transmission of acquired characters[217].

When I previously maintained that the proofs of the transmission of
artificially produced diseases are inconclusive, I had in mind the only
experiments which, as far as I am aware, can be adduced in favour of
the transmission of acquired characters; viz. the experiments of
Brown-Séquard[218] on guinea-pigs. It is well known that he produced
artificial epilepsy in these animals by dividing certain parts of the
central and also the peripheral nervous system. The descendants of the
animals which acquired epilepsy sometimes inherited the disease of
their parents.

These experiments have been since repeated by Obersteiner[219], who has
described them in a very exact and entirely unprejudiced manner. The
fact itself cannot be doubted: it is certain that some of the
descendants of animals in which epilepsy has been artificially
produced, have also themselves suffered from epilepsy in consequence of
the disease of their parents. This fact may be accepted as proved, but
in my opinion we have no right to conclude from it that acquired
characters can be transmitted. Epilepsy is not a morphological
character; it is a disease. We could only speak of the transmission of
a morphological character, if a certain morphological change which was
the cause of epilepsy had been produced by the nervous lesion, and if a
similar change had re-appeared in the offspring, and had produced in
them also the symptoms of epilepsy. But that this really occurs is
utterly unproved; and is even highly improbable. It has only been
proved that many descendants of artificially epileptic parents are
small, weakly, and very soon die; and that others are paralysed in
various parts of the body, i. e. in one or both of the posterior or
anterior extremities; while others again exhibit trophic paralysis of
the cornea leading to inflammation and the formation of pus. In
addition to these symptoms, the descendants in very rare cases exhibit
upon the application of certain stimuli to the skin, a tendency towards
those tonic and clonic convulsions together with loss of consciousness
which constitute the features of an epileptic attack. Out of thirty-two
descendants of epileptic parents only two exhibited such symptoms, both
of them being very weakly, and dying at an early age.

These experiments, although very interesting, do not enable us to
assert that a distinct morphological change is transmitted to the
offspring after having been artificially induced in the parents. The
injury caused by the division of a nerve is not transmitted, and the
part of the brain corresponding to that which was removed from the
parent is not absent from the offspring. The symptoms of a disease
are undoubtedly transmitted, but the cause of the disease in the
offspring is the real question which requires solution. The symptoms
of epilepsy are by no means invariably transmitted; they are in fact
absent from the great majority of cases, and the very small
proportion in which they do occur, exhibit the symptoms of other
diseases in addition to those of epilepsy. The offspring are either
quite healthy (thirteen out of thirty cases) or they suffer from
disturbances of the nervous system, such as the above-mentioned motor
and trophic paralysis,—symptoms which are not characteristic of
epilepsy: however in some of the latter epilepsy is also present.

If therefore we wish to express the matter correctly we must not state
that epilepsy is transmitted to the offspring, but we must express the
facts in the following manner:—animals which have been rendered
epileptic by artificial means, transmit to some of their offspring a
tendency to suffer from various nervous diseases, viz. from motor
paralysis, to a less degree from sensory, and to a high degree from
trophic paralysis; in rare cases, when the symptoms of paralysis are
very marked, epilepsy is also transmitted.

If we now remember that a considerable number of diseases are already
known to be caused by the presence of living organisms in the body, and
that these diseases may be transmitted from one organism to another in
the form of germs, ought we not to conclude from the above-mentioned
facts, that the symptoms are due to an unknown microbe which finds its
nutritive medium in the nervous tissues, rather than to suppose that
they are due to morphological changes, such as a modification of the
histological or molecular structure of certain parts of the nervous
system? At all events, it would be more difficult to understand the
transmission of such a structural change, than the passage of a
bacillus into the sperm- or germ-cell of the parent. There is no
ascertained fact which supports the former assumption, but it is very
probable that the transmission of syphilis, small-pox and
tuberculosis[220] is to be explained by the latter method, although the
bacilli have not yet been detected in the reproductive cells.
Furthermore, this method of transmission has been rigidly proved in the
case of the muscardine disease of the silkworm. At all events we can
understand in this way how it happened that the offspring of
artificially epileptic guinea-pigs were affected with various forms of
nervous disease, a fact which would be quite unintelligible if we
assume the occurrence of a true hereditary transmission of a
morphological character, such as a pathological change in the structure
of some nervous centre.

The manner in which artificial epilepsy becomes manifest after the
operation, is also in favour of the explanation offered above. In the
first place epilepsy does not result from any one single injury to the
nervous system, but it may follow from a variety of different injuries.
Brown-Séquard produced it by removing a portion of the grey matter of
the brain, and by dividing the spinal cord, although the disease also
resulted from a transverse section through half of the latter organ, or
from the section of its anterior or posterior columns alone, or from
simply puncturing its substance. The most striking effects appeared to
follow when the spinal cord was injured in the region between the
eighth dorsal and the second lumbar vertebrae, although the results
were sometimes also produced by the injury of other parts. Epilepsy
also followed the division of the sciatic nerve, the internal
popliteal, and the posterior roots of all nerves which pass to the
legs. The disease never appears at once, but only after the lapse of
some days or weeks, and, according to Brown-Séquard, it is impossible
to conclude that the disease will not follow the operation until after
six or eight weeks have passed without an epileptic attack. Obersteiner
did not witness in any case the first symptoms of the disease for
several days after the division of the sciatic nerve. After the
operation, sensibility decreases over a certain area on the head and
neck, on the same side as the injury. If the animal be pinched in this
region (which is called the epileptic area, ‘zone epileptogène’) it
curves itself round towards the injured side, and violent scratching
movements are made with the hind leg of the same side. After the lapse
of several days or even weeks, these scratching movements which result
from pinching in the above-mentioned area, form the beginning of a
complete epileptic attack. Hence the changes immediately produced by
the division of a nerve are obviously not the direct cause of epilepsy,
but they only form the beginning of a pathological process which is
conducted in a centripetal direction from the nerve to some centre
which is apparently situated in the pons and medulla oblongata,
although, according to others[221], it is placed in the cortex of the
cerebrum. Nothnagel[222] considers that certain changes, the nature of
which is still entirely unknown, but which may be histological or
perhaps solely molecular in character, must be produced, leading to an
increased irritability of the grey matter of the centres concerned.

Nothnagel thinks it possible or even probable that in those cases in
which the division of nerves is followed by epilepsy, a neuritis
ascendens—an inflammation passing along the nerves in a central
direction—is the cause of the changes suggested by him in the epileptic
centre. All our knowledge of bacteria and of the pathological processes
induced by them, seems to indicate that such a neuritis ascendens, as
is assumed by Nothnagel, would render important support to the
hypothesis that the artificial epilepsy is due to infection. But when
we further consider that the offspring of artificially epileptic
animals may themselves become epileptic, although in most cases they
suffer from a variety of other nervous diseases (in consequence of
trophic paralysis), I hardly see how the facts can be rendered
intelligible except by supposing that in these cases of what I may call
traumatic epilepsy, we are dealing with an infectious disease caused by
microbes which find their nutritive medium in the nervous tissues, and
which bring about the transmission of the disease to the offspring by
penetrating the ovum or the spermatozoon.

Obersteiner found that the offspring were more frequently diseased when
the mother was epileptic, rather than the father. This is readily
intelligible when we remember that the ovum contains an immensely
larger amount of substance than the spermatozoon, and can therefore be
more frequently infected by microbes and can contain a greater number
of them.

Of course, I do not mean to assert that epilepsy always depends upon
infection, or upon the presence of microbes in the nervous tissues.
Westphal produced epilepsy in guinea-pigs by striking them once or
twice sharply upon the head: the epileptic attack took place
immediately and was afterwards repeated. It is obvious that the
presence of microbes can have nothing to do with such an attack, but
the shock alone must have caused morphological and functional changes
in the centres of the pons and medulla oblongata, identical with those
produced by microbes in the other cases. Nothnagel also distinctly
expresses the opinion that epilepsy ‘does not depend upon one uniform
and invariable histological change, but that the symptoms which
constitute the disease may in all probability be caused by various
anatomical alterations, provided that they take place in parts of the
pons and medulla which are morphologically and physiologically
equivalent[223].’ Just as a sensory nerve produces the sensation of
pain under various stimuli, such as pressure, inflammation, infection
with the poison of malaria, etc., so various stimuli might cause the
nervous centres concerned to develope the convulsive attack which,
together with its after-effects, we call epilepsy. In Westphal’s case,
such a stimulus would be given by a powerful mechanical shock, in
Brown-Séquard’s experiments, by the penetration of microbes.

However, quite apart from the question of the validity of this
suggestion, we can form no conception as to the means by which an
acquired morphological change in certain nerve-cells—a change which is
not anatomical, and probably not even microscopical, but purely
molecular in nature—can be possibly transferred to the germ-cells: for
this ought to take place in such a manner as to produce in their minute
molecular structure a change which, after fertilization and development
into a new individual, would lead to the reproduction of the same
epileptogenic molecular structure of the nervous elements in the grey
centres of the pons and medulla oblongata as was acquired by the
parent. How is it possible for all this to happen? What substance could
cause such a change in the resulting offspring after having been
transferred to the egg or sperm-cell? Perhaps Darwin’s gemmules may be
suggested; but each gemmule represents a cell, while here we have to do
with molecules or groups of molecules. We must therefore assume the
existence of a special gemmule for each group of molecules, and thus
the innumerable gemmules of Darwin’s theory must be imagined as
increased by many millions. But if we suppose that the theory of
pangenesis is right, and that the gemmules really circulate in the
body, accompanied by other gemmules from the diseased parts of the
brain, and that some of these latter pass into the germ-cells of the
individual,—to what strange results would the further pursuit of this
idea lead? What an incomprehensible number of gemmules must meet in a
single sperm- or germ-cell, if each of them is to contain a
representative of every molecule or group of molecules which has formed
part of the body at each period of ontogeny. And yet such is the
unavoidable consequence of the supposition that acquired molecular
states of certain groups of cells can be transmitted to the offspring.
This supposition could only be rendered intelligible by some theory of
_preformation_[224], such as Darwin’s pangenesis; for the latter theory
certainly belongs to this category. We must assume that each single
part of the body at each developmental stage is, from the first,
represented in the germ-cell as distinct particles of matter, which
will reproduce each part of the body at its appropriate stage as their
turn for development arrives.

I will only briefly indicate some of the inevitable contradictions in
which we are involved by such a theory. One and the same part of the
body must be represented in the germ- or sperm-cell by many groups of
gemmules, each group corresponding to a different stage of development;
for if each part gives off gemmules, which ultimately reproduce the
part in the offspring, it is clear that special gemmules must be given
off for each stage in the development of the part, in order to
reproduce that identical stage. And Darwin quite logically accepts this
conclusion in his provisional hypothesis of pangenesis. But the
ontogeny of each part is in reality continuous, and is not composed of
distinct and separate stages. We imagine these stages as existing in
the continuous course of ontogeny; for here, as in all departments of
nature, we make artificial divisions in order to render possible a
general conception, and to gain fixed points in the continuous changes
of form which have in reality occurred. Just as we distinguish a
sequence of species in the course of phylogeny, although only a gradual
transition, not traversed by sharp lines of demarcation, has taken
place, so also we speak of the stages of ontogeny, although we can
never point out where any stage ends and another begins. To imagine
that each single stage of a part is present in the germ, as a distinct
group of gemmules, seems to me to be a childish idea, comparable to the
belief that the skull of the young St. Laurence exists at Madrid, while
the adult skull is to be found in Rome.

We are necessarily driven to such conceptions if we assume that the
transmission of acquired characters takes place. A theory of
preformation alone affords the possibility of an explanation: an
epigenetic theory is utterly unable to render any assistance in
reaching an interpretation. According to the latter theory, the germ
does not contain any preformed gemmules, but it possesses, as a whole,
such a chemical and molecular constitution that under certain
circumstances, a second stage is produced from it. For example, the two
first segmentation spheres may be regarded as such a second stage;
these again possess such a constitution that a certain third stage, and
no other, can arise from them, forming the four first segmentation
spheres. At each of these stages the spheres produced are peculiar to a
distinct species and a distinct individual. From the third stage a
fourth arises, and so on, until the embryo is developed, and still
later the mature animal which can reproduce itself. No one of the parts
of such an animal was originally present as distinct parts in the egg
from which it was developed, however minute we may imagine these parts
to be. If now an inherited peculiarity shows itself in any organ of the
mature animal, this will be the consequence of the preceding
developmental stages, and if we were able to investigate the molecular
structure of all these stages as far back as the egg-cell, we should
trace back to the latter some minute difference of molecular
constitution which would distinguish it from any other egg-cell of the
same species, and was destined to be the cause of the subsequent
appearance of the peculiarity in the mature animal. It is only by the
aid of some such hypothesis that we can conceive the cause of
hereditary individual differences and the tendencies towards hereditary
diseases. Hereditary epilepsy would be intelligible in this way, that
is, when the disease is congenital and not due to the presence of
microbes, as is presumably the case with artificially induced epilepsy.

The question now arises as to whether we can conceive the communication
of such traumatic and therefore acquired epilepsy to the germ-cells.
This is obviously impossible under the epigenetic theory of development
described above. In what way can the germ-cells be affected by
molecular or histological changes in the pons varolii and medulla
oblongata? Even if we assume, for the sake of argument, that the
central nervous system exercises trophic influences upon the
germ-cells, and that such influences may consist of something more than
variations in nutritive conditions, and may even include the power of
altering the molecular constitution of the germ-plasm in spite of its
usual stability; even if we concede these suppositions, how is it
conceivable that the changes produced would be of the exact nature and
in the exact direction necessary in order to confer upon the germ-plasm
the molecular structure of the first ontogenetic stage of an epileptic
individual? How can the last ontogenetic stage of the ganglion cells in
the pons and medulla of such an individual, stamp upon the germ-plasm
in the germ-cells of the same animal—not indeed the peculiar structure
of the stage itself—but such a molecular constitution as will ensure
the ultimate appearance of epilepsy in the offspring? The theory of
epigenesis does not admit that the parts of the full-grown individual
are contained in the germ as preformed material particles, and
therefore this theory cannot allow that anything is added to the
germ-plasm; but in accepting the above-made supposition, we are
compelled to assume that the molecular structure of the whole of the
germ-plasm is changed to a slight extent.

Nägeli is quite right in maintaining that the solid protoplasm alone,
as opposed to the fluid part, i.e. that part of the protoplasm which
has passed into solution, can act as the bearer of hereditary
tendencies. This appears to be undoubtedly proved by the fact that the
amount of material provided by the male parent for the development of
an embryo is in almost all animals far smaller than the amount provided
by the female parent.

In Mammalia the share contributed by the father probably only forms
about one hundred-billionth part of that contributed by the mother, and
yet nevertheless the influence of the former in heredity is on an
average equal to that exerted by the latter[225]. Now, from the point
of view of epigenesis, no molecule of the brain of an epileptic animal
can reach the germ-cell except in a state of solution, and therefore no
direct increase in the germ-plasm can be referred to such molecules,
quite apart from the fact that such addition, even if possible, could
not be of any value, because the last stage of the epileptic tendency
must be represented in the nerve-cells and nerve-fibres of the diseased
brain, while the first stage ought to be represented in the germ-cell.

It may be safely asserted that according to the theory of epigenesis
the germ-cells cannot be influenced except as regards their nutrition.
Nutritive changes may be imagined to occur through the varying trophic
influence of the nervous system upon the sexual organs, but the
structure of the germ-plasm cannot be altered by mere nutritive
changes, or at all events it cannot be altered in that distinct and
definite direction which is required by the supposed transmission of
acquired epilepsy.

Thus the transmission of artificially produced epilepsy can neither be
explained upon the epigenetic theory, nor upon the theory of
preformation; it can only be rendered intelligible if we suppose that
the appearance of the disease in the offspring depends upon the
introduction and presence of living germs, viz. of microbes. The
supposed transmission of this artificially produced disease is the only
definite instance which has been hitherto brought forward in support of
the transmission of acquired characters. I believe that I have shown
that such support is deceptive, not because there is any uncertainty
about the fact of the transmission itself, but because it is a
transmission which cannot depend upon heredity, and is in all
probability due to infection.

Ever since I began to doubt the transmission of acquired characters, I
have been unable to meet with a single instance which could shake my
conviction. There were many instances in which hereditary transmission
was clearly established, but in none of them was there any reason to
suppose that the characters transmitted were really acquired. For
example, Fritz Müller has recently informed me of an instance in which
he believes that there can be no doubt of the transmission of acquired
characters. His observations are so interesting in several respects
that I will quote them here. He says in his letter, ‘Among the bastards
of two species of _Abutilon_, in which I had never observed hexamerous
flowers, there was a single plant with a few such blossoms. As these
flowers are sterile with the pollen of the same plant, I was obliged to
fertilize it with pollen from another plant bearing only pentamerous
flowers, in order to obtain seeds from the former. For three weeks I
examined all the flowers from a plant grown from such seed, finding 145
pentamerous, 103 hexamerous, and 13 heptamerous flowers. I examined
similarly the flowers of another plant produced from seed obtained from
pentamerous flowers from the same parent plants. There were 454
pentamerous and 6 hexamerous flowers, and hence only 1·3 per cent. of
the latter kind.’

It must certainly be admitted that the large proportion of abnormal
hexamerous flowers depends upon heredity in the instance first quoted;
but the hexamerous condition is not an acquired character; it is merely
the first appearance of a new innate character. It is not due to the
reaction of the vegetable organism under some external stimulus, for it
appeared in a plant exposed to conditions similar to those which acted
upon the other plant which only produced the normal pentamerous
flowers. It must therefore have resulted from the tendencies which were
present in the germ from which the plant itself developed, either as a
spontaneous change in the germ-plasm or through the combination of two
parental germ-plasms—a combination which may lead to the appearance or
the reality of a new character. We know that the germ-plasm of each
individual is not a simple substance, but possesses a very complex
composition, for it consists of a number of ancestral germ-plasms
represented in very different proportions. Now, although we cannot
learn anything directly about the processes of growth of the
germ-plasm, and its resulting ontogenetic stages, yet we do know,
chiefly from observations upon man, that the characters of ancestors
appear in the offspring in very different combinations and in very
different degrees of strength. This may, perhaps, be explained by
assuming that in the union of parental germ-plasms which takes place at
fertilization, the contained ancestral germ-plasms unite in different
ways, and thus come to grow with different strengths. Certain ancestral
germ-plasms will meet and together produce a double effect: other
opposed germ-plasms will neutralize each other; and between these two
extremes all intermediate conditions will occur. And these combinations
will not only take place at fertilization, but also at every stage of
the whole ontogenetic history, for each stage is represented by its
idioplasm, which is itself composed of ancestral idioplasms.

We do not yet know enough to be able to prove in detail the manner in
which new characters may arise from such a combination of different
kinds of germ-plasm. And yet it appears to me that such a view, e.g. in
the case of the variation of buds, is by far the most natural. There is
indeed a single example in which we can, to some extent, understand how
it is that a new character may arise by these means. Certain
canary-birds have a tuft of feathers on the head, but if two such birds
are paired, their descendants are generally bare-headed, instead of
having larger tufts[226]. The formation of a tuft depends upon the fact
that the feathers are scanty and in fact absent from part of the skin
of the head. Now when the scanty plumage of both parents is combined in
the offspring the latter is bare-headed. Hence by the combination of
ancestral characters a new character (bare-headedness) is produced, and
one which is hardly likely to have ever occurred in the ancestors of
existing canaries.

We do not know the causes which have been in operation when a flower
possesses one petal more than the usual number, any more than we can
explain why it is that one star-fish has five and another six rays. We
cannot unravel the details of the mysterious relationship between two
parent germ-plasms, each of which is composed of a countless number of
ancestral germ-plasms from the first and second back to the _n_th
degree. But we can nevertheless maintain in a general way that such
irregularities are the result of this complex struggle between the
germ-plasms in the ovum and the idioplasms in the subsequent stages of
the developing organism, and that they are not the result of external
influences.

If, however, acquired characters are brought forward in connexion with
the question of the transformation of species, the term ‘acquired’ must
only be applied to those characters which do not arise from within the
organism, but which arise as the reaction of the organism under some
external stimulus, most commonly as the consequence of the increased or
diminished use of an organ or part. We have then to learn whether the
altered conditions of life, by forcing an organism to adopt new habits,
can by such means lead directly, and not indirectly through natural
selection, to the transformation of the species; or whether the effects
of increased or diminished use of certain parts, implied by the new
habits, are restricted to the individual itself, and therefore
powerless to effect any direct modification of the species.

Fritz Müller’s observation is also interesting in another respect: it
appears to controvert my views upon heredity as expressed in the theory
of the continuity of the germ-plasm. If a single flower can transmit to
its descendants special peculiarities which were not possessed by its
ancestors, we seem to be driven to the conclusion that the ancestral
germ-plasm has not passed into the flower in question, but that new
germ-plasm has been formed, inasmuch as the new characters are derived
from the flower itself, and not from any of its ancestors. I think,
however, that the observation admits of another interpretation: a
specimen of _Abutilon_ with many hundred flowers is not a single
individual, but a colony consisting of numerous individuals which have
arisen by budding from the first individual developed from the seed.

I have not hitherto considered budding in relation to my theories, but
it is obvious that it is to be explained from my point of view, by
supposing that the germ-plasm which passes on into a budding individual
consists not only of the unchanged idioplasm of the first ontogenetic
stage (germ-plasm), but of this substance altered, so far as to
correspond with the altered structure of the individual which arises
from it—viz. the rootless shoot which springs from the stem or
branches. The alteration must be very slight, and perhaps quite
insignificant, for it is possible that the differences between the
secondary shoots and the primary plant may chiefly depend upon the
changed conditions of development, which takes place beneath the earth
in the latter case, and in the tissues of the plant in the former. Thus
we may imagine that the idioplasm, when it developes into a flowering
shoot, produces at the same time the germ-cells which are found in the
latter. We thus approach an understanding of Fritz Müller’s
observation; for if the whole shoot which produces the flower arises
from the same idioplasm which also forms its germ-cells, we can readily
understand why the latter should contain the same hereditary tendencies
which were previously expressed in the flower which produced them. The
fact that variations may occur in a single shoot depends upon the
changes explained above, which occur in the idioplasm during the course
of its growth, as a result of the varying proportions in which the
ancestral idioplasms may be contained in it.

Fritz Müller’s observation affords a beautiful confirmation of this
view, for if the flower itself transmitted the hexamerous condition to
its germ-cells, we could not understand why some of the extremely rare
hexamerous flowers were produced by the crossing of two pentamerous
flowers, in the control experiment. An explanation of this fact can
only be found in the assumption that the germ-plasm contained in the
mother plant, during its growth and consequent distribution through all
the branches of the colony, became arranged into a combination of
idioplasms, which, whenever it predominated (as it did at certain
places), necessarily led to the formation of hexamerous flowers. I will
not consider here the question as to whether this combination is to be
looked upon as an instance of reversion, or whether it represents
something new. Such a question is of no importance for our present
purpose; but the hexamerous flowers of the control experiment prove, in
my opinion, that germ-plasm containing the requisite combination was
distributed in the mother plant and also existed, but in insufficient
amount, in shoots which did not produce any hexamerous flowers.


           Appendix V. On the Origin of Parthenogenesis[227].

The transformation of heterogeny into pure parthenogenesis has
obviously been produced by other causes as well as by those mentioned
in the main part of this paper. Other and quite different circumstances
have also had a share in its production. Pure parthenogenesis may be
produced without the intermediate condition of heterogeny. Thus, for
example, the pure and exclusive parthenogenesis with which the large
Phyllopod crustacean, _Apus_, is reproduced at most of its habitats,
has not arisen from the loss of previously existent sexual generations,
but simply from the non-appearance of males, accompanied by the
simultaneous acquisition of the power, on the part of the females, of
producing eggs which do not require fertilization. This is proved by
the fact that males occur in certain scattered colonies of this
species, and sometimes they are even present in considerable numbers.
But even if we were not aware of these facts, the same conclusions
might nevertheless have been drawn from the fact that _Apus_ produces
eggs of only one form—viz. resting eggs with hard shells. In every case
in which parthenogenesis has been first introduced in alternation with
sexual reproduction, the resting eggs are produced by the latter
generations, while the parthogenetic generations produce eggs with thin
shells, in which the embryo developes and hatches very rapidly. In this
way parthenogenesis leads to a rapid increase of the colony. In _Apus_
such increase in the number of individuals is gained in an entirely
different manner, viz. by the fact that all the animals become females,
which produce eggs at a very early age, and continue producing them in
increasing fertility for the whole of their life. In this manner an
enormous number of eggs collects at the bottom of the pool inhabited by
the colony, so that after it has dried up, in spite of loss from
various destructive agencies, there will still remain a sufficiency of
eggs to reproduce a numerous colony, as soon as the pool has filled
again.

This form of parthenogenetic reproduction is especially well suited to
the needs of species inhabiting small pools which entirely depend upon
rain-fall, and which may disappear at any time. In these cases the time
during which the colony can live is often too short to permit the
production of several generations even from rapidly developing
summer-eggs. Under these circumstances the pool would often suddenly
dry up before the series of parthenogenetic generations had been run
through, and hence before the appearance of the sexual generation and
resting eggs. In all such cases the colony would be exterminated.

This consideration might lead us to think that Crustacea, such as the
_Daphnidae_, which develope by means of heterogeny, would hardly be
able to exist in small pools filled by the rain; but here also nature
has met the difficulty by another adaptation. As I have shown in a
previous paper[228], the heterogeny of the species of _Daphnidae_ which
inhabit such pools is modified in such a manner, that only the first
generation produced from the resting eggs consists of purely
parthenogenetic females, while the second includes many sexual animals,
so that resting eggs are produced and laid, and the continuance of the
colony is secured a few days after it has been first founded; viz.
after the appearance of the first generation.

But it is also certain that in the _Daphnidae_, heterogeny may pass
into pure parthenogenesis by the non-appearance of the sexual
generations. This seems to have taken place in certain species of
_Bosmina_ and _Chydorus_, although perhaps only in those colonies of
which the continuance is secured for the whole year; viz. those which
inhabit lakes, water-pipes, or wells in which the water cannot freeze.
In certain insects also (e. g. _Rhodites rosae_) pure parthenogenesis
seems to be produced in a similar manner, by the non-appearance of
males.

But the utility which we may look upon as the cause of parthenogenesis
is by no means so clear in all cases. Sometimes, especially in certain
species of Ostracoda, its appearance seems almost like a mere caprice
of nature. In this group of the Crustacea, one species may be purely
parthenogenetic, while a second reproduces itself by the sexual method,
and a third by an alternation of the two methods: and yet all these
species may be very closely allied and may frequently live in the same
locality and apparently with the same habit of life. But it must not be
forgotten that it is only with the greatest difficulty that we can
acquire knowledge about the details of the life of these minute forms,
and that where we can only recognize the appearance of identical
conditions, there may be highly important differences in nutrition,
habits, enemies and the means by which they are resisted, and in the
mode by which the prey is captured—circumstances which may place two
species living in the same locality upon an entirely different basis of
existence. It is not merely probable that this is the case; for the
fact that certain species have modified their modes of reproduction is
in itself a sufficient proof of the validity of the conclusions which
have just been advanced.

The fact that different methods of reproduction may obtain in different
colonies of the same species, although with thoroughly identical
habits, may depend upon differences in the external conditions (as in
_Bosmina_ and _Chydorus_ mentioned above), or upon the fact that the
transition from sexual to parthenogenetic reproduction is not effected
with the same ease and rapidity in all the colonies of the same
species. As long as males continue to make their appearance in a colony
of _Apus_, sexual reproduction cannot wholly disappear. Although we are
unable to appreciate, with any degree of certainty, the causes by which
sex is determined, we may nevertheless confidently maintain that such
determining influences may be different in two widely separated
colonies. As soon, however, as parthenogenesis becomes advantageous to
the species, securing its existence more efficiently than sexual
reproduction, it will not only be the case that the colonies which
produce the fewest males will gain advantage, but within the limits of
the colony itself, those females will gain an advantage which produce
eggs that can develope without fertilization. When the males are only
present in small numbers, it must be very uncertain whether any given
female will be fertilized: if therefore the eggs of such a female
required fertilization in order to develope, it is clear that there
would be great danger of entire failure in this necessary condition. In
other words:—as soon as any females begin to produce eggs which are
capable of development without fertilization, from that very time a
tendency towards the loss of sexual reproduction springs into
existence. It seems, however, that the power of producing eggs which
can develope without fertilization is very widely distributed among the
Arthropoda.


          Appendix VI. W. K. Brooks’ Theory of Heredity[229].

The only theory of heredity which, at any rate in one point, agrees
with my own, was brought forward two years ago by W. K. Brooks of
Baltimore[230]. The point of agreement lies in the fact that Brooks
also looks upon sexual reproduction as the means employed by nature in
order to produce variation. The manner in which he supposes that the
variability arises is, however, very different from that suggested in
my theory, and our fundamental conceptions are also widely divergent.
While I look upon the continuity of the germ-plasm as the foundation of
my theory of heredity, and therefore believe that permanent hereditary
variability can only have arisen through some direct change in the
germ-plasm effected by external influences, or following from the
varied combinations which are due to the mixture of two individually
distinct germ-plasms at each act of fertilization, Brooks, on the other
hand, bases his theory upon the transmission of acquired characters,
and upon the idea which I have previously called ‘the cyclical
development of the germ-plasm.’

Brooks’ theory of heredity is a modification of Darwin’s pangenesis,
for Brooks also assumes that minute gemmules are thrown off by each
cell in the body of the higher organisms; but such gemmules are not
emitted always, and under all circumstances, but only when the cell is
subjected to unaccustomed conditions. During the persistence of the
ordinary conditions to which it is adapted, the cell continues to
perform its special functions as part of the body, but as soon as the
conditions of life become unfavourable and its functions are disturbed,
the cell ‘throws off minute particles which are its germs or gemmules.’

These gemmules may then pass into any part of the organism; they may
penetrate the ova in the ovary, or may enter into a bud, but the male
germ-cells possess a special power of attracting them and of storing
them up within themselves.

According to Brooks, variability arises as a consequence of the fact
that each gemmule of the sperm-cell unites, during fertilization, with
that part of the ovum which, in the course of development, is destined
to become a cell corresponding to that from which the gemmule has been
derived.

Now, when this cell developes in the offspring, it must, as a hybrid,
have a tendency to vary. The ova themselves, as cells, are subject to
the same laws; and the cells of the organism will continue to vary
until one of the variations is made use of by natural selection. As
soon as this is the case, the organism becomes, _ipso facto_, adapted
to its conditions; and the production of gemmules ceases, and with it
the manifestation of variability itself, for the cells of the organism
then derive the whole of their qualities from the egg, and being no
longer hybrid, have no tendency to vary. For the same reason the ova
themselves will also cease to vary, and the favourable variation will
be transmitted from generation to generation in a stereotyped
succession, until unfavourable conditions arise, and again lead to a
fresh disposition to vary.

In this way Brooks[231] attempts to mediate between Darwin and Lamarck,
for he assumes, on the one hand, that external influences render the
body or one of its parts variable, while, on the other hand, the nature
of the successful variations is determined by natural selection. There
is, however, a difference between the views of Brooks and Darwin,
although not a fundamental difference. Darwin also holds that the
organism becomes variable by the operation of external influences, and
he further assumes that changes acquired in this way can be
communicated to the germ and transmitted to the offspring. But
according to his hypothesis, every part of the organism is continually
throwing off gemmules which may be collected in the germ-cells of the
animal, while, according to Brooks, this only takes place in those
parts which are placed under unfavourable conditions or the function of
which is in some way disturbed. In this manner the ingenious author
attempts to diminish the incredible number of gemmules which, according
to Darwin’s theory, must collect in the germ-cells. At the same time he
endeavours to show that those parts must always vary which are no
longer well adapted to the conditions of life.

I am afraid, however, that Brooks is confounding two things which are
in reality very different, and which ought necessarily to be treated
separately if we wish to arrive at correct conclusions: viz., the
adaptation of a part of the body to the body itself, and its adaptation
to external conditions. The first of these adaptations may exist
without the second. How can those parts become variable which are badly
adapted to the external conditions, but are nevertheless in complete
harmony with the other parts of the body? If the conditions of life of
the cells which constitute the part in question must become
unfavourable, in order that the gemmules which produce variation may be
thrown off, it is obvious that such a result would not occur in the
case mentioned above. Suppose, for example, that the spines of a
hedgehog are not sufficiently long or sharply pointed to afford
protection to the animal, how could such an unfavourable development
afford the occasion for the throwing off of gemmules, and a resulting
variability of the spines, inasmuch as the epidermic tissue in which
these structures arise, remains under completely normal and favourable
conditions, whatever length or sharpness the spines may attain? The
conditions of the epidermis are not unfavourably affected because, as
the result of short and blunt spines, the number of hedgehogs is
reduced to far below the average. Or consider the case of a brown
caterpillar which would gain great advantage by becoming green; what
reason is there for believing that the cells of the skin are placed in
unfavourable conditions, because, in consequence of the brown colour,
far more caterpillars are detected by their enemies, than would have
been the case if the colour were green? And the case is the same with
all adaptations. Harmony between the parts of the organism is an
essential condition for the existence of the individual. If it is
wanting, the individual is doomed; but such harmony between any one
part and all others, i. e. proper nutrition for each part, and adequate
performance of its proper function, can never be disturbed by the fact
that the part in question is insufficiently adapted to the outer
conditions of life. According to Darwin, all the cells of the body are
continually throwing off gemmules, and against such an assumption no
similar objection can be raised. It can only be objected that the
assumption has never been proved, and that it is extremely improbable.

A further essential difference between Darwin’s theory of pangenesis
and Brooks’ hypothesis lies in the fact that Brooks holds that the male
and female germ-cells play a different part, and that they tend to
become charged with gemmules in different degrees, the egg-cell
containing a far smaller number than the sperm-cell. According to
Brooks the egg-cell is the conservative principle which brings about
the permanent transmission of the true characters of the race or
species, while he believes that the sperm-cell is the progressive
principle which causes variation.

The transformation of species is therefore believed to take place, for
the most part, as follows:—those parts which are placed in unfavourable
conditions by the operation of external influences, and which have
varied, throw off gemmules which reach the sperm-cells, and the latter
by fertilization further propagate the variation. An increase of
variation is produced because the gemmules which reach the egg through
the sperm-cell may unite or conjugate with parts of the former which
are not the exact equivalents of the cells from which the gemmules
arose, but only very nearly related to them. Brooks calls this
‘hybridization,’ and he concludes that, just as hybrids are more
variable than pure species, so such hybridized cells are also more
variable than other cells.

The author has attempted to work out the details of his theory with
great ingenuity, and as far as possible to support his assumptions by
facts. Moreover, it cannot be denied that there are certain facts which
seem to indicate that the male germ-cell plays a different part from
that taken by the female germ-cell in the formation of a new organism.

For example, it is well known that the offspring of a horse and an ass
is different when the male parent is a horse from what it is when the
male parent is an ass. A stallion and a female ass produce a hinny
which is more like a horse, while a male ass and a mare produce a mule
which is said to be more like an ass[232]. I will refrain from
considering here the opinion of several authors (Darwin, Flourens, and
Bechstein) that the influence of the ass is stronger in both cases,
only predominating to a less extent when the ass is the female parent;
and I will for the sake of brevity accept Brooks’ opinion that in these
cases the influence of the father is greater than that of the mother.
Were this so in all cross-breeding between different species and in all
cases of normal fertilization, we should be compelled to conclude that
the influences of the male and female germ-plasms upon the offspring
differ at any rate in strength. But this is by no means always the
case, for even in horses the reverse may occur. Thus it is stated that
certain female race-horses have always transmitted their own
peculiarities, while others allowed those of the stallion to
preponderate.

In the human species the influence of the mother preponderates quite as
often as that of the father, although in many families most of the
children may take after either parent. There is nevertheless hardly any
large family in which all the children take after the same parent. If
we now try to explain the preponderating influence of one parent by the
supposition of a greater strength in hereditary power, without first
inquiring after some deeper cause, I think the only conclusion
warranted by the facts before us is that this power is rarely or never
equal in both of the conjugating germ-cells, but that even within the
same species, sometimes the male and sometimes the female is the
stronger, and that the strength may even vary in the different
offspring of the same individuals, as we so frequently see in human
families. The egg-cells of the same mother which ripen one after the
other, and also the sperm-cells of the same father, must therefore
present variations in the strength of their hereditary power. It is
then hardly to be wondered at that the relative hereditary power of the
germ-cells in different species should vary, although we cannot as yet
understand why this should be the case.

It would not be very difficult to render these facts intelligible in a
general way by an appeal to physiological principles. The quantity of
germ-plasm contained in a germ-cell is very minute, and together with
the idioplasms of the various ontogenetic stages to which it gives
rise, it must be continually increased by assimilation during the
development of the organism. If now this power of assimilation varied
in intensity, a relatively rapid growth of the idioplasm derived from
one of the parents would ensue, and with it the preponderance of the
hereditary tendencies of the parent in question. Now, it is obvious
that no two cells of the same kind are entirely identical, and hence
there must be differences in their powers of assimilation. Thus the
varying hereditary powers of the egg-cells produced from the same ovary
become explicable, and still more easily the varying powers of the
germ-cells produced in the ovaries or testes of different individuals
of the same species; most easily of all the differences observable in
this respect between the germ-cells of different species.

Of course, this hereditary power is always relative, as may be easily
proved by cross-breeding between different species and races. Thus when
a fantail pigeon is crossed with a laugher, the characters of the
former preponderate, but when crossed with a pouter the characters of
the latter preponderate[233]. The facts afforded by cross-breeding
between hybrids and one of the pure parent species, together with a
consideration of the resulting degree of variability, seem to me to be
even more unfavourable to Brooks’ view. They appear to me to admit of
an interpretation different from that brought forward by him; and when
he proceeds to make use of secondary sexual characters for the purpose
of his theory, I believe that his interpretation of the facts can be
easily controverted. It is hardly possible to conclude that variability
is due to the male parent, because the males in many species of animals
are more variable, or deviate further from the original type, than the
females. It is certainly true that in many species the male sex has
taken the lead in processes of transformation, while the female sex has
followed, but there is no difficulty in finding a better explanation of
the fact than that afforded by the assumption ‘that something within
the animal compels the male to lead and the female to follow in the
evolution of new breeds.’ Brooks has with great ingenuity brought
forward certain instances which cannot be explained with perfect
confidence by Darwin’s theory of sexual selection, but this hardly
justifies us in considering the theory to be generally insufficient,
and in having recourse to a theory of heredity which is as complicated
as it is improbable. The whole idea of the passage of gemmules from the
modified parts of the body into the germ-cells is based upon the
unproved assumption that acquired characters can be transmitted. The
idea that the male germ-cell plays a different part from that of the
female, in the construction of the embryo, seems to me to be untenable,
especially because it conflicts with the simple observation that upon
the whole human children inherit quite as much from the father as from
the mother.

------------------------------------------------------------------------

                 Footnotes for Appendices for Essay V.

Footnote 199:

  Appendix to page 257.

Footnote 200:

  l. c., p. 118.

Footnote 201:

  l. c., p. 118.

Footnote 202:

  Appendix to page 258.

Footnote 203:

  l. c., p. 137.

Footnote 204:

   Compare Brücke, ‘Farbenwechsel des Chamäleon.’ Wien. Sitzber. 1851.
  Also Leydig, ‘Die in Deutschland lebenden Saurier,’ 1872.

Footnote 205:

  ‘Philosophical Transactions,’ vol. cxlviii. 1858, pp. 627-644.

Footnote 206:

  Adler, ‘Beiträge zur Naturgeschichte der Cynipiden,’ Deutsche entom.
  Zeitschr. XXI., 1877, p. 209; and by the same author, ‘Ueber den
  Generationswechsel der Eichen-Gallwespen,’ Zeitschr. f. wiss. Zool.,
  Bd. XXXV. 1880, p. 151.

Footnote 207:

  Beyerinck, ‘Beobachtungen über die ersten Entwicklungsphasen einiger
  Cynipidengallen,’ Verhandl. d. Amsterd. Akad. d. Wiss. Bd. XXII. 1883.

Footnote 208:

  l. c., p. 144.

Footnote 209:

  [It is now known that many such caterpillars are actually modified in
  colour by their surroundings, but the process appears to be indirect
  and secondarily acquired by the operation of natural selection, like
  that of the change of colour in the chamaeleon, frogs, fish, etc.;
  although the stimulus of light acts upon the eyes of the latter
  animals and upon the skin of the caterpillar. See the seventh Essay
  (pp. 394-397) for a more detailed account.—E. B. P.]

Footnote 210:

  l. c., p. 150.

Footnote 211:

  In order to make the case as simple as possible, I assume that the
  insectivorous bird feeds upon a single species of insect, and that
  the insect is only attacked by a single species of bird.

Footnote 212:

  English Edition, translated by D’Arcy W. Thompson, B.A. London, 1883,
  p. 509 et seqq.

Footnote 213:

  Appendix to page 260.

Footnote 214:

  Ch. Darwin, ‘On the fertilization of Orchids by Insects.’ London,
  1877.

Footnote 215:

  Compare Hermann Müller, ‘Die Befruchtung der Blumen durch Insekten
  und die gegenseitigen Anpassungen beider.’ Leipzig, 1873. See also
  many articles by the same author in ‘Kosmos,’ and other periodicals.
  These later articles are included in the English translation by
  D’Arcy W. Thompson.

Footnote 216:

  ‘Lectures on the Physiology of Plants,’ translated by H. Marshall
  Ward, Oxford, 1887, p. 47.

Footnote 217:

  Appendix to page 267.

Footnote 218:

  Brown-Séquard, ‘Researches on epilepsy; its artificial production in
  animals and its etiology, nature, and treatment.’ Boston, 1857. Also
  various papers by the same author in ‘Journal de physiologie de
  l’homme,’ Tome I and III, 1858, 1860, and in ‘Archives de physiologie
  normale et pathologique,’ Tome I-IV, 1868-1872.

Footnote 219:

  ‘Oesterreichische medicinische Jahrbücher.’ Jahrgang, 1875, p. 179.

Footnote 220:

  A direct transmission of the germs of disease through the
  reproductive cells has lately been rendered probable in the case of
  tuberculosis, for the bacilli have been found in tubercles in the
  lungs of an eight-months’ fœtal calf, the mother being affected at
  the time with acute tuberculosis. However it is not impossible that
  infection may have arisen through the placenta. See ‘Fortschritte der
  Medicin,’ Bd. III, 1885, p. 198.

Footnote 221:

  Compare Unvericht, ‘Experimentelle und klinische Untersuchungen über
  die Epilepsie.’ Berlin, 1883. With regard to the question of
  hereditary transmission, the part of the brain in which the epileptic
  centre is placed is of no importance.

Footnote 222:

  Compare Ziemssen’s Handbuch der spec. Pathologie und Therapie.’ Bd.
  XII. 2. Hälfte; Artikel ‘Epilepsie und Eklampsie.’ Leipzig, 1877.

Footnote 223:

  l. c., p. 269.

Footnote 224:

  It is generally known that the earlier physiologists believed in what
  was called the ‘evolutionary theory,’ or the ‘theory of
  preformation.’ This assumes that the germ contains, in a minute form,
  the whole of the fully-developed animal. All the parts of the adult
  are preformed in the germ, and development only consists in the
  growth of these parts and their more perfect arrangement. This theory
  was generally accepted until the middle of the last century, when
  Kaspar Friedrich Wolff brought forward the theory of ‘epigenesis,’
  which since that time has been the dominant one. This assumes that no
  special parts of the germ are preformations of certain parts of the
  fully-developed animal, and that these latter arise by a series of
  changes in the germ, which gradually gives rise to them. In modern
  times the theory of preformation has been revived in a less crude
  form, as is shown by the ideas of Nägeli, and by Darwin’s
  ‘pangenesis.’—A. W., 1888.

Footnote 225:

  Nägeli, l. c. p. 110.

Footnote 226:

  See Darwin, ‘The Variation of Animals and Plants under
  Domestication.’ 1875. Vol. I. p. 311.

Footnote 227:

  Appendix to page 290.

Footnote 228:

  Weismann, ‘Naturgeschichte der Daphnoiden,’ Zeitschrift f. wiss.
  Zool. XXIII. 1879.

Footnote 229:

  Appendix to page 277.

Footnote 230:

  Compare W. K. Brooks, ‘The Law of Heredity, a Study of the Cause of
  Variation, and the Origin of living Organisms.’ Baltimore, 1883.

Footnote 231:

  l. c., p. 82.

Footnote 232:

  This seems to be the general opinion (see the quotation from Huxley
  in Brooks’ ‘Heredity,’ p. 127); but I rather doubt whether there is
  such a constant difference between mules and hinnies. Furthermore, I
  cannot accept the opinion that mules always resemble the ass more
  than the horse. I have seen many mules which bore a much stronger
  likeness to the latter. I believe that it is at present impossible to
  decide whether there is a constant difference between mules and
  hinnies, because the latter are very rarely seen, and because mules
  are extremely variable. I attempted to decide the question last
  winter by a careful study of the Italian mules, but I could not come
  across a single hinny. These hybrids are very rarely produced,
  because it is believed that they are extremely obstinate and
  bad-tempered. I afterwards saw two true hinnies at Professor Kühn’s
  Agricultural Institute at Halle. These hinnies by no means answered
  to the popular opinion, for they were quite tractable and
  good-tempered. They looked rather more like horses than asses,
  although they resembled the latter in size. In this case it was quite
  certain that one parent was a stallion and the other a female ass.—A.
  W. 1889.

Footnote 233:

  Darwin, ‘Variation of Animals and Plants under Domestication,’ 1875,
  Vol. II. p. 41.

------------------------------------------------------------------------






                                  VI.

                   ON THE NUMBER OF POLAR BODIES AND
                    THEIR SIGNIFICANCE IN HEREDITY.

                                 1887.






------------------------------------------------------------------------




   ON THE NUMBER OF POLAR BODIES AND THEIR SIGNIFICANCE IN HEREDITY.


                                PREFACE.

The following paper stands in close relation to a series of short
essays which I have published from time to time since the year 1881.
The first of these treated of ‘The Duration of Life,’ and the last of
‘The Significance of Sexual Reproduction.’ The present essay is most
intimately connected with that upon ‘The Continuity of the Germ-plasm,’
and has, in fact, grown out of the explanation of the meaning of polar
bodies in the animal egg, brought forward in that essay. The
explanation rested upon a trustworthy and solid foundation, as I am now
able to maintain with even greater confidence than at that time. It
rested upon the idea that in the egg-cell, a cell with a high degree of
histological differentiation, two different kinds of nuclear substance
exert their influence, one after the other. But continued investigation
has shown me that the explanation built upon this idea is only correct
in part, and that it does not exhaust the full meaning of the formation
of polar bodies. In the present essay I hope to complete the
explanation by the addition of essential elements, and I trust that, at
the same time, I shall succeed in throwing new light upon the
mysterious problems of sexual reproduction and parthenogenesis.

It is obvious that this essay can only contain an attempt at an
explanation, an hypothesis, and not a solution which is above
criticism, like the results of mathematical calculation. But no
biological theory of the present day can escape a similar fate, for the
mathematical key which opens the door leading to the secrets of life
has not yet been found, and a considerable period of time must elapse
before its discovery. But although I can only offer an hypothesis, I
hope to be able to show that it has not been rashly adopted, but that
it has grown in a natural manner from the secure foundation of
ascertained facts.

Nothing impresses the stamp of truth upon an hypothesis more than the
fact that its light renders intelligible not only those facts for the
explanation of which it has been framed, but also other and more
distantly related groups of phenomena. This seems to me to be the case
with my hypothesis, since the interpretation of polar bodies and the
ideas derived from it unite from very different points of view, the
facts of reproduction, heredity and even the transformation of species,
into a comprehensive system, which although by no means complete, is
nevertheless harmonious, and therefore satisfactory.

Only the most essential elements of the new facts which form the
foundation of the views developed in this essay will be briefly
mentioned. My object is to show all the theoretical bearings of these
new facts, not to describe them in technical detail. Such a description
accompanied by the necessary figures will shortly be given in another
place[234].

                                                             A. W.

      Freiburg I. Br., _May 30, 1887_.




------------------------------------------------------------------------

                   ON THE NUMBER OF POLAR BODIES, &c.

                               CONTENTS.

                                                              PAGE

        I. PARTHENOGENETIC AND SEXUAL EGG                      339

           The process of the formation of polar bodies very   339
           widely distributed

           The significance of polar bodies according to       340
           Minot, Balfour, and van Beneden

           My hypothesis of the removal of the histogenetic    341
           part of the nucleus

           Confirmation by the discovery of polar bodies in    345
           parthenogenetic eggs

           Parthenogenetic eggs form only _one_ polar body,    346
           while eggs requiring fertilization  form _two_

           Parthenogenesis depends upon the fact that the      348
           part of the nucleus which is expelled from sexual
           eggs  in the second polar body, remains in the egg

           History of this discovery                           349


       II. SIGNIFICANCE OF THE SECOND POLAR BODY               352

           Refutation of Minot’s theory                        353

           The second division of the nuclear spindle          355
           involves a reduction of the ancestral germ-plasms

           The theoretical necessity for such reduction        356

           Phyletic origin of the germ-plasms in existing      357
           species

           The necessary reduction takes place by a special    358
           form of nuclear division

           The division which causes this reduction has        360
           probably been already observed

           Van Beneden’s and Carnoy’s observations             360

           Two different physiological effects of              364
           karyokinesis

           Significance of direct nuclear division             365

           Arguments in support of the view that the division  367
           of the egg-nucleus which causes reduction must
           occur at the end of ovogenetic development

           Such nuclear division is to be found in the         368
           formation of the second polar body

           History of the origin of this view                  368


      III. THE FOREGOING CONSIDERATIONS APPLIED TO THE MALE    370
           GERM-CELLS

           The male germ-cells also require division in order  370
           to reduce the ancestral germ-plasms

           The germ-plasms of the parents must be contained    370
           in the germ-plasm of the offspring

           Advantages which the egg gains by the late          371
           occurrence of the ‘reducing division’

           The causes of unequal division in the formation of  373
           polar bodies

           These causes do not apply to the sperm-cell         373

           Different kinds of nuclear division occur in        375
           spermatogenesis

           Some of these may be interpreted as ‘reducing       375
           divisions’

           The paranucleus (‘Nebenkern’) of spermatogenesis    376
           probably contains the histogenetic nucleoplasm


       IV. THE FOREGOING CONSIDERATIONS APPLIED TO PLANTS      377


        V. CONCLUSIONS AS REGARDS HEREDITY                     378

           The germ-cell of an individual contains an unequal  378
           combination of hereditary tendencies

           Dissimilarity between the offspring of the same     379
           parents

           Identity of twins produced from a single egg        380


       VI. RECAPITULATION                                      383

------------------------------------------------------------------------




                                  VI.

                ON THE NUMBER OF POLAR BODIES AND THEIR
                       SIGNIFICANCE IN HEREDITY.

                   I. Parthenogenetic and Sexual Egg.

Hitherto no value has been attached to the question whether an animal
egg produces one or two polar bodies. Several observers have found two
such bodies in many different groups of animals, both high and low in
the scale of organization. In certain species only one has been
observed, in others again three, four, or five (e. g. Bischoff, in the
rabbit). Many observers did not even record the number of polar bodies
found by them, and simply spoke of ‘polar bodies.’ As long as their
formation was looked upon as a process of secondary physiological
importance—as an ‘excretion,’ or a ‘process of purification,’ or even
as the ‘excreta’ (!) of the egg, as a ‘rejuvenescence of the nucleus,’
or of mere historical interest as a reminiscence of ancestral
processes, without any present physiological meaning—so long was it
unnecessary to attach any importance to the number of these bodies, or
to pay special attention to them. Of all the above-mentioned views, the
one which explained polar bodies as a mere reminiscence of ancestral
processes seemed to be especially well founded. Ten years ago we were
far from being able to prove that polar bodies occurred in all animal
eggs, and even in 1880, Balfour said in his excellent ‘Comparative
Embryology,’ ‘It is very possible, not to say probable, that such
changes [the formation of polar bodies] are universal in the animal
kingdom, but the present state of our knowledge does not justify us in
saying so[235].’

Even at the present day we are not, strictly speaking, justified in
making this assertion, for polar bodies have not yet been proved to
occur in certain groups of animals, such as reptiles and birds; but
they have been detected in the great majority of the large groups of
the animal kingdom, and wherever they have been looked for with the aid
of our modern highly efficient appliances, they have been found[236].

A deeper insight into the process of fertilization has above all led to
a closer study of antecedent phenomena.

O. Hertwig[237] and Fol[238] showed that the formation of polar bodies
was connected with a division of the nuclear substance of the egg.
Hertwig and Bütschli[239] then proved that the body expelled from the
egg possessed the nature of a cell, and thus led the way to the view
that the formation of polar bodies is a process of cell-division,
although a very unequal one. Even then there was no reason for
attaching any special importance to the number of these bodies; nor
should we have such a reason if we agreed with Minot[240],
Balfour[241], and van Beneden in ascribing a high physiological
significance to this process, and assumed that the expelled polar body
is the male part of the previously hermaphrodite egg-cell. We should
not know in what proportion the quantities of the ‘male’ and ‘female’
parts were present, and it would therefore be impossible to decide, _a
priori_, whether the ‘male’ part had to be removed from the body of the
egg-cell in one, two, or more portions.

Even after the view that the nuclear substance is the essential element
in fertilization had gained ground—a view chiefly due to Strasburger’s
investigations on the process of fertilization in Phanerogams—and after
Hertwig’s opinion had been confirmed, that the process of fertilization
is essentially the conjugation of nuclei, even then there appeared to
be no reason why the number of divisions undergone by the nucleus of
the mature egg should be looked upon as an essential feature.

This was the state of the subject at the time when I first made an
attempt to ascertain the meaning of the formation of polar bodies. I
based my views upon the idea, which was just then gaining ground, that
Nägeli’s idioplasm was to be sought for in the nucleus, and that the
nucleoplasm must therefore contain the substance which determines the
form and functions of the cell. Hence it followed that the
germ-plasm—the substance which determines the course of embryonic
development—must be identified with the nucleoplasm of the egg-cell.
The conception of germ-plasm was brought forward by me before the
appearance of Nägeli’s work[242] which is so rich in fertile ideas; and
germ-plasm does not exactly coincide with Nägeli’s idioplasm[243].
Germ-plasm is only a certain kind of idioplasm—viz. that contained in
the germ-cell—and it is the most important of all idioplasms, because
all the other kinds are merely the results of the various ontogenetic
stages into which it developes. I attempted to show that the molecular
structure in these ontogenetic stages into which the germ-plasm
developes would become more and more unlike that of the original
structure of this substance, until it finally attains a highly
specialized character at the end of embryonic development,
corresponding to the production of specialized histological elements.
It did not seem to me to be conceivable that the specialized idioplasm
contained in the nuclei of the tissue cells could re-transform itself
into the initial stage of the whole developmental series—that it could
give up its specialized character and re-assume the generalized
character of germ-substance. I will not repeat the reasons which
induced me to adopt this opinion; they still seem to me to be
conclusive. But let the above-mentioned theory be once accepted, and
there follows from it another interesting conclusion concerning the
germ-cell, or at least concerning those germ-cells which, like most
animal eggs, possess a specific histological character. For obviously,
such a character presupposes the existence of an idioplasm with a
considerable degree of histological specialization, which must be
contained in the nucleus of the egg-cell. We know, on the other hand,
that when its growth is complete, after the formation of yolk and
membranes, the egg contains germ-plasm, for it is capable of developing
into an embryo. We have therefore, as it were, two natures in a single
cell, which become manifest one after the other, and which, according
to our fundamental conception, can only be explained by the presence of
two different idioplasms, which control the egg-cell one after the
other, and determine its processes of development. At first a
nucleoplasm leading to histological specialization directs the
development of the egg and stamps upon it a specific histological
character; and then germ-plasm takes its place, and compels the egg to
undergo development into an embryo. If then the histogenetic or
ovogenetic nucleoplasm of the egg-cell can be derived from the
germ-plasm, but cannot be re-transformed into it (for the specialized
can be derived from the generalized, but not the generalized from the
specialized), we are driven to the conclusion that the germ-plasm,
which is already present in the youngest egg-cell, first of all
originates a specific histogenetic or ovogenetic nucleoplasm which
controls the egg-cell up to the point at which it becomes mature; that
its place is then taken by the rest of the unchanged nucleoplasm
(germ-plasm), which has in the meantime increased by growth; and that
the former is removed from the egg in the form of polar bodies—a
removal which has been rendered possible by the occurrence of nuclear
division. Hence the formation of polar bodies signified, in my opinion,
the removal of the ovogenetic part of the nucleus from the mature
egg-cell. Such removal was absolutely necessary, if it is impossible
that the ovogenetic nucleoplasm can be re-transformed into germ-plasm.
Hence the former substance cannot be made use of after the maturation
of the egg, and it must even be opposed to the commencement of
embryonic development, for it is impossible that the egg can be
controlled by two forces of different kinds in the same manner as it
would have been by one of them alone. I therefore concluded that the
influence of the ovogenetic idioplasm must be removed before embryonic
development can take place. In this way it seemed to me that not only
the ordinary cases of ovogenetic and embryonic development became more
easily intelligible, but also the rarer cases in which one and the same
species produces two kinds of eggs—‘summer and winter eggs.’ Such eggs
not only differ in size but also in the structure of yolk and
membranes, although identical animals are developed from each of them.
This result presupposes that the nucleus in both eggs contains
identical germ-plasm, while the formation of different yolks and
membranes requires the supposition that their nucleoplasm is different,
inasmuch as the two eggs differ greatly in histological character.

The fact that equal quantities are separated during nuclear division,
led me to conclude further that the expulsion of ovogenetic nucleoplasm
can only take place when the germ-plasm in the nucleus of the egg-cell
has increased by growth up to a point at which it can successfully
oppose the ovogenetic nuclear substance. But we do not know the
proportion which must obtain between the relative quantities of two
different nuclear substances in order that nuclear division may be
induced; and thus, by this hypothesis at least, we could not conclude
with certainty as to the necessity for a single or a double division of
the egg. It did not seem to be altogether inconceivable that the
ovogenetic nucleoplasm might be larger in amount than the germ-plasm,
and that it could only be completely removed by means of two successive
nuclear divisions. I admit that this supposition caused me some
uneasiness; but since nothing was known which could have enabled us to
penetrate more deeply into the problem, I was satisfied, for the time
being, in having found any explanation of the physiological value of
polar bodies; leaving the future to decide not only whether such
explanation were valid, but also whether it were exhaustive. The
explanation seems to have found but little favour with some of our
highest authorities. Hensen[244] does not consider that my reasons for
the distinction between germ-plasm and histogenetic nucleoplasm are
conclusive, and it may be conceded that this objection was perhaps, at
that time, well founded. O. Hertwig does not mention my hypothesis at
all in his work on embryology[245], although he states in the preface:
‘Among current problems I have chiefly taken into consideration the
views which seem to me to be most completely justified, but I have not
left unmentioned the views which I cannot accept.’ Minot’s hypothesis
is discussed by Hertwig, but Bütschli’s[246] is preferred by him,
although these two hypotheses are not strictly opposed to each other;
for the former is a purely physiological, the latter a purely
morphological explanation. I desire to lay especial stress upon the
fact that my hypothesis is simply a logical consequence from the
conclusion that the nuclear substance determines the nature of a cell.
How this takes place is quite another question, which need not be
discussed here. If it is only certain that the nature of a cell is thus
determined, it follows that a cell with a certain degree of
histological specialization must contain a nucleoplasm corresponding to
the specialization. But the mature egg also contains germ-plasm, and
there are only two possibilities by which these facts can be explained:
either the ovogenetic nucleoplasm is capable of re-transformation into
germ-plasm, or it is incapable of such re-transformation. Now, quite
apart from the arguments which might be advanced in favour of one of
these two possibilities, the fact that a body is undoubtedly expelled
from the mature egg seems to me of importance, while it is of even
greater importance that this body contains nucleoplasm from the
germ-cell.

It may be thought that the process, as supposed by me, is without
analogy, but such a conclusion is wrong, for during every embryonic
development there are numerous cell-divisions in which unequal
nucleoplasms are separated from one another, and in all these cases we
cannot imagine any way in which the process can take place, except by
supposing that the two kinds of nucleoplasm were previously united in
the mother-cell, although their differentiation probably took place
only a short time before cell-division. Perhaps the new facts which
will be mentioned presently, and the views derived from them, will make
my hypothesis upon the histogenetic nucleoplasm of the germ-cells
appear in a more favourable light to the authorities above-named.

My hypothesis has at all events the one merit that it has led me to
fruitful investigations.

If the formation of polar bodies really means the removal of ovogenetic
nucleoplasm from the mature egg, they must also be found in
parthenogenetic eggs; inasmuch as the latter possess a specific
histological structure equal to that found in eggs requiring
fertilization. If, therefore, it were possible to observe the formation
of polar bodies in eggs which develope parthenogenetically, such an
observation would not form a proof of the validity of my
interpretation; but it would be a fact which harmonized with it, and
negatived a suggestion which, if confirmed, would have been fatal to
the hypothesis. Minot, Balfour, and van Beneden, from the point of view
afforded by their theories, were compelled to suppose that polar bodies
are wanting in parthenogenetic eggs; and the facts which were known at
that time favoured such an opinion, for in spite of many attempts, no
one had ever succeeded in proving the formation of these bodies by
parthenogenetic eggs.

During the summer of 1885 I first succeeded in ascertaining that a
single polar body is expelled from the parthenogenetic summer-egg of
one of the _Daphnidae_,—_Polyphemus oculus_[247]. Thus my
interpretation of the process in question received support, while it
seemed to me that Minot’s interpretation of polar bodies had been
refuted; for if these bodies are formed in the parthenogenetic eggs of
a single species, just as in eggs which require fertilization, it
follows that the expulsion of polar bodies cannot signify the removal
of the male element from the egg.

The desire to throw light upon the significance of polar bodies has
been the only cause of my investigation. At the same time I hoped by
this means to gain further knowledge as to the nature of
parthenogenesis.

In the third part of the essay on ‘The Continuity of the Germ-plasm’
(see p. 225) I attempted to make clear the nature of parthenogenesis,
and I arrived at the conclusion that the difference between an egg
which is capable of developing without fertilization, and another which
requires fertilization, must lie in the quantity of nucleoplasm present
in the egg. I supposed that the nucleus of the mature parthenogenetic
egg contained nearly twice as much germ-plasm as that contained in the
sexual egg, just before the occurrence of fertilization; or, more
correctly, I believed that the quantity of nucleoplasm which remains in
the egg, after the expulsion of the polar bodies, is the same in both
eggs, but that the parthenogenetic egg possesses the power of doubling
this quantity by growth, and thus produces from within itself the same
quantity of germ-plasm as that contained in the sexual egg after the
addition of the sperm-nucleus in fertilization.

This was only an hypothesis, and the considerations which had led to it
depended, as far as they went into details, upon assumptions; but the
fundamental view that the _quantity_ of the nucleus decides whether
embryonic development takes place with or without fertilization seemed
to me, even at that time, to be correct, and to be a conclusion
required by the facts of the case. Indeed, I thought it not unlikely
that its validity might be proved by direct means: I pointed out that a
comparison of the quantities of the nuclei in parthenogenetic and
sexual eggs, if possible in the same species, would enable us to decide
the question (_l. c._, p. 234).

I had thus set myself the task of making this comparison. The result of
this investigation was to show that, as already mentioned, polar bodies
are formed in parthenogenetic eggs. But even the first species
successfully investigated revealed a further fact, which, if proved to
be wide-spread and characteristic of all parthenogenetic eggs, was
certain to be of extreme importance:—the maturation of the
parthenogenetic egg is accompanied by the expulsion of _one_ polar
body, or, as we might express it in another way, the substance of the
female pronucleus is only _once_ divided, and not _twice_, as in the
sexual eggs of so many other animals. If this difference between
parthenogenetic and sexual eggs was shown to be general, then the
foundations of my hypothesis would indeed have been proved to be sound.
The quantity of nuclear substance decides whether the egg is capable of
undergoing embryonic development. This quantity is twice as large in
the parthenogenetic as in the sexual egg. I had, however, been mistaken
in a matter of detail; for the difference in the quantities of nuclear
substance is not produced by the expulsion of two polar bodies, and the
reduction of the nuclear substance to a quarter of its original amount,
in both eggs, while the parthenogenetic egg then doubles its nuclear
substance by growth; but the difference is produced because the
reduction of nuclear substance originally present is less in one case
than it is in the other. In the parthenogenetic egg the nuclear
substance is only reduced to one-half by a single division; in the
sexual egg it is reduced to a quarter by two successive divisions. It
is an obvious conclusion from this fact, if proved to be wide-spread,
that the significance of the first polar body must be different from
that of the second. Only one polar body can signify the removal of
ovogenetic nucleoplasm from the mature egg, and the second is obviously
a reduction of the germ-plasm itself to half of its original amount.
This very point seemed to me to be of great importance, because, as I
had foreseen long ago, and as will be shown later on, the theory of
heredity forces us to suppose that every fertilization must be preceded
by a reduction of the ancestral idioplasms present in the nucleus of
the parent germ-cell, to one-half of their former number.

But before the full bearing of the phenomena could be considered, it
was necessary to ascertain how far they were of general occurrence.
There were two ways in which this might be achieved, and in which it
was possible to prove that parthenogenetic eggs expel only one polar
body, while sexual eggs expel two. We might attempt to observe the
phenomena of maturation in both kinds of eggs in a species which
reproduces itself by the parthenogenetic as well as the sexual method.
This would be the simplest way in which the question could be decided,
if it were possible to make such observations on a sufficient number of
species. But the other method was also open, a method which would have
been the only one, if we did not know of any animals with two kinds of
reproduction. We might attempt to investigate the phenomena of
maturation in a large number of parthenogenetic eggs, if possible from
different groups of animals, and we might compare the results with the
facts which are already certain concerning the expulsion of polar
bodies from the sexual eggs of so many species.

I have followed both methods, and by means of the second I have arrived
already, indeed some time ago, at the certain conclusion that the
above-mentioned difference is really general and without exception. The
first polar body only is formed in all the parthenogenetic eggs which I
investigated, with the valuable assistance of my pupil, Mr. Ischikawa
of Tokio. On the other hand, an extensive examination of the literature
of the subject convinced me that there is not a single undoubted
instance of the expulsion of only one polar body from eggs which
require fertilization, and that there are very numerous cases known
from almost all groups of the animal kingdom in which it is perfectly
certain that two polar bodies are formed, one after the other. A number
of the older observations cannot be relied upon, for the presence of
two polar bodies is mentioned without any explanation as to whether
they are expelled from the egg one after the other, or whether they
have merely resulted from the division of a single body after its
expulsion. In parthenogenetic eggs two polar bodies are also formed in
most cases, but they arise from the subsequent division of the single
body which separates from the egg. But such subsequent division is only
of secondary importance as far as the egg itself is concerned, and is
also unimportant in the interpretation of the process. The essential
nature of the process is to be found in the fact that the nucleus of
the egg-cell only divides once when parthenogenesis occurs, but twice
when fertilization is necessary, and it is of no importance whether the
expelled part of the nucleus of the cell-body atrophies at once, or
after it has undergone division. We have, therefore, to distinguish
between primary and secondary polar bodies. If this distinction is
recognized, and if we leave out of consideration all doubtful cases
mentioned in literature, such a large number of well-established
observations remain, that the existence of two primary polar bodies in
sexual eggs, and neither a smaller nor a larger number, may be
considered as proved.

Hence follows a conclusion which I believe to be very significant,—the
difference between parthenogenetic and sexual eggs lies in the fact
that in the former only one primary polar body is expelled, while two
are expelled from the latter. When, in July, 1886, I published a short
note[248] on part of the observations made upon parthenogenetic eggs, I
confined myself to facts, and did not mention this conclusion. I took
this course simply because I did not wish to bring it forward until I
had made sufficient observations in the first of the two ways described
above. I had hoped to be able to offer all the proofs that can be
obtained before undertaking to publish the far-reaching consequences
which would result from the above-mentioned conclusion. Unfortunately
the material with which I had hoped to quickly settle the matter,
proved less favourable than I had expected. Many hundred sections
through freshly laid winter-eggs of _Bythotrephes longimanus_ were made
in vain; they did not yield the wished for evidence, and although
continued investigation of other material has led to better results,
the proofs are not yet entirely complete.

I should not therefore even now have brought forward the
above-mentioned conclusion, if another observer had not alluded to this
idea, referring to my observations and also to a new discovery of his
own. In a recent number of the ‘Biologische Centralblatt,’
Blochmann[249] gives an account of his continued observations upon the
formation of polar bodies. It is well known that this careful observer
had previously shown that polar bodies do occur in the eggs of insects,
although they had not been found before. Blochmann proved that they are
found in the representatives of three different orders, so that we may
indeed ‘confidently hope to find corresponding phenomena in other
insects.’ This discovery is most important, and it was naturally very
welcome to me, as I had for a long time ascribed a high physiological
importance to the process of the formation of polar bodies, and it
would not be in accordance with such a view if the process was entirely
wanting from whole classes of animals. To fill up this gap in our
knowledge, and to give the required support to my theoretical views, I
had proposed to one of my pupils, Dr. Stuhlmann[250], that he should
work out the maturation of the eggs of insects; and it is a curious
ill-luck that he, like many other observers, did not succeed in
observing the expected expulsion of polar bodies, in spite of the great
trouble he had taken. It may be that the species selected for
investigation were unfavourable: at all events, we cannot now doubt
that a division of the egg-nucleus is quite universal among insects,
for Blochmann, in his latest contribution to the subject, proves that
the _Aphidae_ also form polar bodies. He examined the winter-eggs of
_Aphis aceris_, and ascertained that they form two polar bodies, one
after the other. Even in the viviparous _Aphidae_, thin sections
revealed the presence of a polar body, though Blochmann could not trace
all the stages of its development. It appears that the polar body is
here preserved for an exceptional period, and its presence can still be
proved when the blastoderm has been formed, and sometimes when
development is even further advanced. Skilled observers of recent
times, such as Will and Witlaczil, have not been able to find a polar
body in the parthenogenetic eggs of the _Aphidae_, and Blochmann’s
proof of its existence seems to me to be of especial value, because the
eggs of _Aphidae_ are in many respects so unusually reduced; for
instance, the primary yolk is absent and the egg-membrane is completely
deficient, so that we might have expected that if polar bodies are ever
absent, they would be wanting in these animals—that is, if they were of
no importance, or at any rate of only secondary importance.

Hence the presence of polar bodies in _Aphidae_ is a fresh confirmation
of their great physiological importance. As bearing upon the main
question dealt with in this essay, Blochmann’s observations have an
especial interest, because only one polar body was found in the
parthenogenetic eggs of _Aphis_, while the sexual eggs normally produce
two. The author rightly states that this result is in striking
accordance with my results obtained from the summer-eggs of different
_Daphnidae_, and he adds the remark,—‘It would be of great interest to
know whether these facts are due to the operation of some general law.’
To this remark I can now reply that there is indeed such a law: not
only in the parthenogenetic eggs of _Daphnidae_, but also, as I have
since found, in those of the Ostracoda and Rotifera[251], only one
primary polar body is formed, while two are formed in all eggs destined
for fertilization.

Before proceeding to the conclusions which follow from this fact, I
will at once remove a difficulty which is apparently presented by the
eggs which may develope with or without fertilization. I refer to the
well-known case of the eggs of bees. It might be objected to my theory
that the same egg cannot be prepared for development in more than one
out of the two possible ways; it might be argued that the egg either
possesses the power of entering upon two successive nuclear divisions
during maturation, and in this case requires fertilization; or the egg
may be of such a nature that it can only enter upon one such division
and can therefore form only one polar body, and in that case it is
capable of parthenogenetic development. Now there is no doubt, as I
pointed out in my paper on the nature of parthenogenesis[252], that in
the bee the very same egg may develope parthenogenetically, which under
other circumstances would have been fertilized. Bessel’s[253]
experiments, in which young queens were rendered incapable of flight,
and were thus prevented from fertilization, have shown that all the
eggs laid by such females develope into drones (males) which are well
known to result from parthenogenetic development. On the other hand,
bee-keepers have long known that young queens which are fertilized in a
normal manner continue for a long time to lay eggs which develope into
females, that is to say, which have been fertilized. Hence the same
eggs, viz. those which are lowest in the oviducts and are therefore
laid first, develope parthenogenetically in the mutilated female, but
are fertilized in the normal female. The question therefore arises as
to the way in which the eggs become capable of adapting themselves to
the expulsion of two polar bodies when they are to be fertilized, and
of one only when fertilization does not take place.

But perhaps the solution of this problem is not so difficult as it
appears to be. If we may assume that in eggs which are capable of two
kinds of development the second polar body is not expelled until the
entrance of a spermatozoon has taken place, the explanation of the
possibility of parthenogenetic development when fertilization does not
occur would be forthcoming. Now we know, from the investigations of O.
Hertwig and Fol, that in the eggs of _Echinus_ the two polar bodies are
even formed in the ovary, and are therefore quite independent of
fertilization, but in this and other similar cases a parthenogenetic
development of the egg never takes place. There are, however,
observations upon other animals which point to the fact that the first
only and not the second polar body may be formed before the
spermatozoon penetrates into the egg. It can be easily understood why
it is that entirely conclusive observations are wanting, for hitherto
there has been no reason for any accurate distinction between the first
and the second polar body. But in many eggs it appears certain that the
second polar body is not expelled until the spermatozoon has
penetrated. O. Schultze, the latest observer of the egg of the frog, in
fact saw the first polar body alone extruded from the unfertilized egg:
a second nuclear spindle was indeed formed, but the second polar body
was not expelled until after fertilization had taken place. A very
obvious theory therefore suggests itself:—that while the formation of
the second polar body is purely a phenomenon of maturation in most
animal eggs, and is independent of fertilization,—in the eggs of a
number of other animals, on the other hand, and especially among
Arthropods, the formation of the second nuclear spindle is the result
of a stimulus due to the entrance of a spermatozoon. If this suggestion
be confirmed, we should be able to understand why parthenogenesis
occurs in certain classes of animals wherever the external conditions
of life render its appearance advantageous, and further, why in so many
species of insects a sporadic parthenogenesis is observed, viz. the
parthenogenetic development of single eggs (Lepidoptera). Slight
individual differences in the facility with which the second nuclear
spindle is formed independently of fertilization would in such cases
decide whether an egg is or is not capable of parthenogenetic
development. As soon, however, as the second nuclear spindle is formed,
parthenogenesis becomes impossible. The nuclear spindle which gives
rise to the second polar body, and that which initiates segmentation,
are two entirely different things, and although they contain the same
quantity, and the same kind of germ-plasm, a transformation of the one
into the other is scarcely conceivable. This conclusion will be
demonstrated in the following part of the essay.

             II. The Significance of the Second Polar Body.

I have already discussed the physiological importance of the first
polar body, or rather of the first division undergone by the nucleus of
the egg, and I have explained it as the removal of ovogenetic nuclear
substance which has become superfluous and indeed injurious after the
maturation of the egg. I do not indeed know of any other meaning which
can be ascribed to this process, now that we know of the occurrence of
a first division of the nucleus in parthenogenetic as well as in sexual
eggs. A part of the nucleus must thus be removed from both kinds of
eggs, a part which was necessary to complete their growth, and which
then became superfluous and at the same time injurious. In this respect
the observations of Blochmann[254] upon the eggs of _Musca vomitoria_
seem to me to be very interesting. Here the two successive divisions of
the nuclear spindle arising from the egg-nucleus take place, but true
polar bodies are not expelled, and the two nuclei corresponding to them
(one of which divides once more) are placed on the surface of the egg,
surrounded by an area free from yolk granules; and they break up at a
later period. The essential point is obviously to eliminate from the
egg-cell the influence of nucleoplasm which has been separated from the
egg-nucleus as the first polar body; and this condition is satisfied
whether the elimination is brought about by a process of true
cell-division, as is the rule in the eggs of most animals, or by the
division and removal of part of the egg-nucleus alone. The occurrence
of the latter method of elimination certainly constitutes a still
further proof of the physiological importance of the process, and this,
taken together with the universal occurrence of polar bodies in all
eggs—parthenogenetic and sexual—forces us to conclude that the process
must possess a definite significance. No one of the various attempts
which have been made to explain the significance of polar bodies
generally is applicable to the _first_ polar body except that which I
have attempted.

But the case is different with the significance of the _second_ nuclear
division, or the _second_ polar body. Here it might perhaps be possible
to return to the view brought forward by Minot, Balfour, and van
Beneden, and to consider the removal of this part of the nucleus as the
expulsion of the male part of the previously hermaphrodite egg-cell.
The second polar body is only expelled when the egg is to be
fertilized, and at first sight it appears to be quite obvious that such
a preparation of the egg for fertilization must depend upon its
reduction to the female state. I believe however that this is not the
case, and am of opinion that the process has an entirely different and
much deeper meaning.

How can we gain any conception of this supposed hermaphroditism of the
egg-cell, and its subsequent attainment of the female state? What are
the essential characteristics of the male and female states? We know of
female and male individuals, among both animals and plants: their
differences consist essentially in the fact that they produce different
kinds of reproductive cells; in part they are of a secondary nature,
being adaptations of the organism to the functions of reproduction;
they are intended to attract the other sex, or to ensure the meeting of
the two kinds of reproductive cells, or to enable the fertilized egg to
develope and sometimes to guide the development of the offspring until
it has reached a certain period of growth. But all these differences,
however great they may sometimes be, do not alter the essential nature
of the organism. The blood corpuscles of man and woman are the same,
and so are the cells of their nerves and muscles; and even the sexual
cells, so different in size, appearance, and generally also in motile
power, must contain the same fundamental substance, the same idioplasm.
Otherwise the female germ-cell could not transmit the male characters
of the ancestors of the female quite as readily as the female
characters, nor could the male germ-cell transmit the female quite as
readily as the male characters of the ancestors of the male. It is
therefore clear that the nuclear substance itself is not sexually
differentiated.

I have already previously pointed out that the above-mentioned facts of
heredity contain the disproof of Minot’s theory, inasmuch as the
egg-cell transmits male as well as female characters. Strasburger[255]
has also raised a similar objection. I consider this objection to be
quite conclusive, for there does not seem to be any way in which the
difficulty can be met by the supporters of the theory. The difficulty
could indeed be evaded until we came to know that the essential part of
the polar body is nuclear substance, and that the latter must be
regarded as idioplasm,—as the substance which is the bearer of
heredity. It might have been maintained that the male part, removed
from the egg, consists only in a condition, perhaps comparable to
positive or negative electricity; and that this condition is present in
the substance of the polar body, so that the removal of the latter
would merely signify a removal of the unknown condition. I do not mean
to imply that any of those who have adopted Minot’s theory have had any
such vague ideas concerning this process, but even if any one were
ready to adopt it, he would be unable to make any use of the idea. He
would not be able to support the theory in this way, for we now know
that nuclear substance is removed with the polar body, and this fact
requires an explanation which cannot be afforded by the theory, if we
are right in believing that the expelled nuclear substance is not
merely the indifferent bearer of the unknown principle of the male
condition, but hereditary substance. I therefore believe that Minot’s,
Balfour’s, and van Beneden’s hypothesis, although an ingenious attempt
which was quite justified at the time when it originated, must be
finally abandoned.

My opinion of the significance of the second polar body is shortly
this,—a reduction of the germ-plasm is brought about by its formation,
a reduction not only in quantity, but above all in the complexity of
its constitution. By means of the second nuclear division the excessive
accumulation of different kinds of hereditary tendencies or germ-plasms
is prevented, which without it would be necessarily produced by
fertilization. With the nucleus of the second polar body as many
different kinds of idioplasm are removed from the egg as will be
afterwards introduced by the sperm-nucleus; thus the second division of
the egg-nucleus serves to keep constant the number of different kinds
of idioplasm, of which the germ-plasm is composed during the course of
generations.

In order to make this intelligible a short explanation is necessary.

From the splendid series of investigations on the process of
fertilization, commenced by Auerbach and Bütschli, and continued by
Hertwig, Fol, Strasburger, van Beneden, and many others, and from the
theoretical considerations brought forward by Pflüger, Nägeli, and
myself, at least one certain result follows, viz. that there is an
hereditary substance, a material bearer of hereditary tendencies, and
that this substance is contained in the nucleus of the germ-cell, and
in that part of it which forms the nuclear thread, which at certain
periods appears in the form of loops or rods. We may further maintain
that fertilization consists in the fact that an equal number of loops
from either parent are placed side by side, and that the segmentation
nucleus is composed in this way. It is of no importance, as far as this
question is concerned, whether the loops of the two parents coalesce
sooner or later, or whether they remain separate. The only essential
conclusion demanded by our hypothesis is that there should be complete
or approximate equality between the quantities of hereditary substance
derived from either parent. If then the germ-cells of the offspring
contain the united germ-plasms of both parents, it follows that such
cells can only contain half as much paternal germ-plasm as was
contained in the germ-cells of the father, and half as much maternal
germ-plasm as was contained in the germ-cells of the mother. This
principle is affirmed in a well-known calculation made by breeders of
animals, who only differ from us in their use of the term ‘blood’
instead of the term germ-plasm. Breeders say that half of the ‘blood’
of the offspring has been derived from the father and the other half
from the mother. The grandchild similarly derives a quarter of its
‘blood’ from each of the four grandparents, and so on.

Let us imagine, for the sake of argument, that sexual reproduction had
not been introduced into the animal kingdom, and that asexual
reproduction had hitherto existed alone. In such a case, the germ-plasm
of the first generation of a species which enters upon sexual
reproduction must still be entirely homogeneous; the hereditary
substance must, in each individual, consist of many minute units, each
of which is exactly like the other, and each of which contains within
itself the tendency to transmit, under certain circumstances, the whole
of the characters of the parent to a new organism—the offspring. In
each of the offspring of such a first generation, the germ-plasms of
two parents will be united, and every germ-cell contained in the
individuals of this second sexually produced generation will now
contain two kinds of germ-plasm—one kind from the father, and the other
from the mother. But if the total quantity of germ-plasm present in
each cell is to be kept within the pre-determined limits, each of the
two ancestral germ-plasms, as I may now call them, must be represented
by only half as many units as were contained in the parent germ-cells.

In the third sexually produced generation, two new ancestral
germ-plasms would be added by fertilization to the two already present,
and the germ-cells of this generation would therefore contain four
different ancestral germ-plasms, each of which would constitute a
quarter of the total quantity. In each succeeding generation the number
of the ancestral germ-plasms is doubled, while their quantities are
reduced by one half. Thus in the fifth sexually produced generation,
each of the sixteen ancestral germ-plasms will only constitute 1/16 of
the total quantity; in the sixth, each of the thirty-two ancestral
germ-plasms, only 1/32, and so on. The germ-plasm of the tenth
generation would be composed of 1024 different ancestral germ-plasms,
and that of the n^{th} of 2^n. By the tenth generation each single
ancestral germ-plasm would only form 1/1024 of the total quantity of
germ-plasm contained in a single germ-cell. We know nothing whatever of
the length of time over which this process of division of the ancestral
germ-plasms may have endured, but even if it had continued to the
utmost possible limit—so far indeed that each ancestral germ-plasm was
only represented by a single unit—a time would at last come when any
further division into halves would cease to be possible; for the very
conception of a unit implies that it cannot be divided without the loss
of its essential nature, which in this case constitutes it as the
hereditary substance.

In the diagram represented in Fig. I. I have tried to render these
conclusions intelligible. In generation I. each paternal and maternal
germ-plasm is still entirely homogeneous, and does not contain any
combination of different hereditary qualities, but the germ-plasm of
the offspring is made up of equal parts of two kinds of germ-plasm. In
the second generation this latter germ-plasm unites with another
derived from other parents, which is similarly composed of two
ancestral germ-plasms, and the resulting third generation now contains
four different ancestral germ-plasms in its germ-cells, and so on. The
diagram only indicates the fusion of ancestral germ-plasms as far as
the offspring of the fourth generation, the germ-cells of which contain
sixteen different ancestral germ-plasms. If we imagine the germ-plasm
units to be so large that there is only room for sixteen of them in the
nuclear thread, the limits of division would-be reached in the fifth
generation, and any further division into halves of the ancestral
germ-plasms would be impossible.

Now however minute the units may be, there is not the least doubt that
the limits of possible division have been long since reached by all
existing species, for we may safely assume that no one of them has
acquired the sexual method of reproduction within a small number of
recent generations. All existing species must therefore now contain as
many different kinds of ancestral germ-plasms as they are capable of
containing; and the question arises,—How can sexual reproduction now
proceed without a doubling of the quantity of germ-plasm in each
germ-cell, with every new generation?

There is only one possible answer to such a question:—sexual
reproduction can proceed by a reduction in the _number_ of ancestral
germ-plasms, a reduction which is repeated in every generation.

[Illustration: Fig. I.]

This _must_ be so: the only question is, how and when does the supposed
reduction take place.

Inasmuch as the germ-plasm is seated, according to our theory, in the
nucleus, the necessary reduction can only be produced by nuclear
division; and quite apart from any observation which has been already
made, we may safely assert that there _must_ be a form of nuclear
division in which the ancestral germ-plasms contained in the nucleus
are distributed to the daughter-nuclei in such a way that each of them
receives only half the number contained in the original nucleus. After
Roux’s[256] elaborate review of the whole subject, we need no longer
doubt that the complex method of nuclear division, hitherto known as
karyokinesis, must be considered not merely as a means for the division
of the total quantity of nuclear substance, but also for producing a
division of the quantity and quality of each of its single elements. In
by far the greater number of instances the object of this division is
obviously to effect an equal distribution of nuclear substance in the
two daughter-nuclei, so that each of the different qualities contained
in the mother-nucleus is transferred to the two daughter-nuclei. This
interpretation of ordinary karyokinesis is less uncertain than perhaps
at first sight it may appear to be. We cannot, it is true, directly see
the ancestral germ-plasms, nor do we even know the parts of the nucleus
which are to be looked upon as constituting ancestral germ-plasm; but
if Flemming’s original discovery of the longitudinal division of the
loops lying in the equatorial plane of the nuclear spindle is to have
any meaning at all, its object must be to divide and distribute the
different kinds of the minutest elements of the nuclear thread as
equally as possible. It has been ascertained that the two halves
produced by the longitudinal splitting of each loop never pass into the
same daughter-nucleus, but always in opposite directions. The essential
point cannot therefore be the division of the nucleus into absolutely
equal quantities, but it must be the distribution of the different
qualities of the nuclear thread, without exception, in both
daughter-nuclei. But these different qualities are what I have called
the ancestral germ-plasms, i.e. the germ-plasms of the different
ancestors, which must be contained in vast numbers, but in very minute
quantities, in the nuclear thread. The supposition of a vast number is
not only required by the phenomena of heredity but also results from
the comparatively great length of the nuclear thread: furthermore it
implies that each of them is present in very small quantity. The vast
number together with the minute quantity of the ancestral germ-plasms
permit us to conclude that they are, upon the whole, arranged in a
linear manner in the thin thread-like loops: in fact the longitudinal
splitting of these loops appears to me to be almost a proof of the
existence of such an arrangement, for without this supposition the
process would cease to have any meaning.

This is the only kind of karyokinesis which has been observed until
recently; but if the supposed nuclear division leading to a reduction
in the number of ancestral germ-plasms has any real existence, there
must be yet another kind of karyokinesis, in which the primary
equatorial loops are not split longitudinally, but are separated
without division into two groups, each of which forms one of the two
daughter-nuclei. In such a case the required reduction in the number of
ancestral germ-plasms would take place, for each daughter-nucleus would
receive only half the number which was contained in the mother-nucleus.

Now there is more evidence for the existence of this second kind of
karyokinesis than the fact that it is demanded by my theory; for I
believe that it has been already observed, although it has not been
interpreted in this sense.

It is very probable that this is true of van Beneden’s[257] observation
on the egg of _Ascaris megalocephala_: he found that the nuclear
division which led to the formation of the polar body differs from the
ordinary course of karyokinesis, in that the plane of division is at
right angles to that usually assumed. Carnoy[258] has confirmed this
observation in its main features, and he has made the further
observation that out of the eight nuclear loops which are found at the
equator of the spindle, four are removed with the first polar body, and
that half of the remaining four are removed with the second polar body.
The first of these two divisions would have to be looked upon as a
reduction, if it is certain that each of the eight nuclear loops
consists of different ancestral germ-plasms; but this assumption is
impossible, although on the other hand it cannot be _directly_
disproved: for we are not able to see the ancestral germ-plasms. But it
must nevertheless be maintained that the removal of the first four
loops does not imply a reduction in the number of ancestral germ-plasms
in the nucleus; because, as I have already argued, two successive
divisions of the number of ancestral germ-plasms into halves is
inconceivable; and because the first polar body is also present in
parthenogenetic eggs in which such division into halves cannot take
place. But the karyokinetic process can readily be looked upon as a
removal of ovogenetic nucleoplasm, for we know from the observations of
Flemming and Carnoy, that, under certain circumstances, subsequent
divisions may occur, involving an increase in the number of nuclear
loops to double their number. These subsequent divisions of course take
place in the daughter-nuclei. This fact proves, as I think, that there
are nuclei in which the same ancestral germ-plasm occurs in two
different loops: but such loops, identical as regards the composition
of their ancestral germ-plasms, may very well contain different
ontogenetic stages of this substance. This will be the case in the
instance alluded to, if four loops of the first nuclear spindle are to
be looked upon as ovogenetic nucleoplasm, and the four others as
germ-plasm. It is therefore unnecessary to regard the first division of
the egg-nucleus as a ‘reducing division’: it may be looked upon as an
‘equal division’[259] entirely analogous to the kind of division which,
in my opinion, directs the development of the embryo. This conclusion
would receive direct proof if it were possible to show that the eight
loops of the first division have arisen by the longitudinal splitting
of four _primary_ loops: for a longitudinal splitting of the nuclear
thread would be the means by which the different ontogenetic stages of
the germ-plasm could be separated from one another, without leading to
any reduction in the number of ancestral germ-plasms in the
daughter-nuclei. Thus I have previously attempted to prove that the
ontogenetic development of the egg must be connected with a progressive
transformation of the nucleoplasm during successive nuclear divisions,
and this transformation will very frequently (but not always) occur in
such a way that the different qualities of the nucleoplasm are
separated from one another by the nuclear division. The nucleoplasm of
the daughter-nuclei will be identical if the two daughter-cells are to
potentially contain corresponding parts of the embryo; as for instance
the first two segmentation spheres of the egg of the frog, which
according to Roux[260] correspond to the right and left halves of the
future animal. But the nucleoplasm must be unequal if the products of
division are to develope into different parts of the embryo. In both
cases, however, karyokinesis is connected with a longitudinal splitting
of the nuclear threads, and we may conclude from this fact (which is
also confirmed by the phenomena of heredity) that all such nuclei,
whether they have entered upon the same or different ontogenetic
transformations of their nucleoplasm, are identical as regards the
ancestral germ-plasm which they contain. During the whole process of
segmentation and the entire development of the embryo, the total number
of ancestral germ-plasms which were at first contained in the
germ-plasm of the fertilized egg-cell must still be contained in each
of the succeeding cells.

Thus no objection can be raised against the view that the four loops of
the first polar body contain the ovogenetic nucleoplasm, that is to
say, an idioplasm which contains the total number of ancestral
germ-plasms, but at an advanced and highly specialized ontogenetic
stage.

The formation of the second polar body may be rightly considered as a
‘reducing division,’ as a division leading to the expulsion of half the
number of the different ancestral germ-plasms, in the form of two
nuclear loops, for no reason can be alleged in support of the
assumption that the four loops of the second nuclear spindle are made
up of identical pairs. Furthermore the facts of heredity require the
assumption that the greatest possible number of ancestral germ-plasms
is accumulated in the germ-plasm of each germ-cell, and thus that the
small number of loops not only means an increase in quantity but a
multiplication in the number of different ancestral germ-plasms present
in each of them. If this conclusion be correct, there can be no doubt
that the second division of the egg-nucleus means a reduction in the
above-mentioned sense.

But there are yet other observations which, if correct, must also be
considered as ‘reducing divisions.’ I refer to all those cases in which
the longitudinal splitting of the loops is either entirely wanting, or
does not occur until after the loops have left the equator of the
spindle and have moved towards the poles. In both instances the bearing
upon the question would be the same, for only half the number of
primary loops would reach each pole in either case. If therefore the
primary loops are not made up of identical pairs, it follows that the
two daughter-nuclei can only contain half the number of ancestral
germ-plasms which were contained in the mother-nucleus. Whether the
loops divide on their way to the poles or at the poles themselves, no
difference will be brought about in the number of ancestral germ-plasms
which they contain, for this number can neither increase nor diminish.
The _quantity_ of the different ancestral germ-plasms can alone be
increased in this way. I am here referring to observations made by
Carnoy[261] on the cells which form the spermatozoa in various
Arthropods. It must be admitted, however, that these divisions cannot
be regarded as ‘reducing divisions,’ if Flemming’s[262] suggestion be
confirmed, that in all these observations the fact has been overlooked
that the equatorial loops are not primary but secondary, and that they
have arisen from the longitudinal splitting of the nuclear thread
during previous stages of nuclear division. But this point can only be
decided by renewed investigation. Although many excellent results have
been obtained in the subject of karyokinesis, there is still very much
to be learnt before our knowledge is complete; and this is not to be
wondered at when we remember the great difficulties in the way of
observation which are chiefly raised by the minute size of the objects
to be investigated. Flemming’s most recent publications prove that we
are still in the midst of investigation, and that highly interesting
and important processes have hitherto escaped attention. A secure basis
of facts is only very gradually obtained, and there are still many
conflicting opinions upon the details of this process. I should
therefore consider it to be entirely useless, from my point of view, to
enter into a critical examination of everything known about all the
details of karyokinesis. I am quite content to have shown how it may be
imagined that the reduction required by my theory takes place during
nuclear division; and at the same time to have pointed out that there
are already observations which may be interpreted in this sense. But
even if I am mistaken in this interpretation, the theoretical necessity
for a reduction in the number of ancestral germ-plasms, a reduction
repeated in every generation, seems to me to be so securely founded
that the processes by which it is effected _must_ take place, even if
they are not supplied by the facts already ascertained. There must be
two kinds of karyokinesis according to the different physiological
effect of the process. First, a karyokinesis by means of which all the
ancestral germ-plasms are equally distributed in each of the two
daughter-nuclei after having been divided into halves: secondly, a
karyokinesis by means of which each daughter-nucleus receives only half
the number of ancestral germ-plasms possessed by the mother-nucleus.
The former may be called ‘equal division,’ the latter ‘reducing
division.’ Of course these two processes, which differ so greatly in
their effects, must also be characterized by morphological differences,
but we cannot assume that the latter are necessarily visible. Just as,
during the division of the first and second nuclear spindle in the egg
of _Ascaris megalocephala_, karyokinesis takes, upon the whole, the
same morphological course, although we must ascribe different
physiological meanings to the two processes of division,—so it may be
in other cases. The ‘reducing division’ must be always accompanied by a
reduction of the loops to half their original number, or by a
transverse division of the loops (if such division ever occurs);
although reduction can only occur when the loops are not made up of
identical pairs. And it will not always be easy to decide whether this
is the case. On the other hand, the form of karyokinesis in which a
longitudinal splitting of the loops takes place _before_ they separate
to form the daughter-nuclei must always, as far as I can see, be
considered as an ‘equal division.’ In the accompanying figures II and
III, diagrams are given illustrating these two forms of karyokinesis,
but I do not mean to imply that it is impossible to imagine any other
form in which they may occur.

In Figure II a nuclear spindle is seen at _A_, and at its equatorial
zone there are twelve primary loops. The transverse cross-lines and
other markings on the loops indicate that they are composed of
different ancestral germ-plasms. The loops are shaded differently in
order to render the diagram clear. At _B_ six of the loops are seen to
have moved to either pole, so that the figure is a representation of
the ‘reducing division.’ Figure III is a diagrammatic representation of
‘equal division.’ The six loops at the equatorial zone of _A_ are shown
by different cross-lining and shading to be composed of different
ancestral germ-plasms. The loops split longitudinally in a direction
indicated by the longitudinal line upon each of them. In _B_ the halves
of the loops are seen to have moved to the opposite poles of the
spindle, so that there are not only six loops at each pole, but also
all the six combinations of ancestral germ-plasms.

[Illustration: Figs. II, III.]

Perhaps some may be inclined to look upon direct nuclear division as a
‘reducing division,’ but I believe that such a view would be incorrect.
It is only approximately true that the nuclear thread is divided into
two halves of equal quantity by direct division, and exact equality
would only happen as it were accidentally; so that we cannot speak of a
perfectly equal distribution of the ancestral germ-plasm in the two
daughter-nuclei. But the ‘reducing division’ must obviously effect an
exactly regular and uniform distribution of the ancestral germ-plasms,
although this does not imply that every ancestral germ-plasm of the
mother-nucleus would be represented in each of the two daughter-nuclei.
But if out of e.g. eight nuclear loops at the equatorial plane, four
pass into one, and the other four into the other daughter-nucleus, each
of the latter will contain an equal number of ancestral germ-plasms,
although different ones. This is indeed part of the foundation of the
theory, for the ‘reducing division’ must remove exactly half of the
original number of ancestral germ-plasms, and precisely the same number
must be replaced at a later period by the sperm-nucleus. This could
hardly be achieved with sufficient precision by direct nuclear division.

I now come to inquire whether the expulsion of the second polar body is
in reality, as I have already maintained, a reduction in the number of
ancestral germ-plasms present in the nucleus of the egg. The view
itself is sufficiently obvious, and it would supply an explanation of
the meaning of the process which is still greatly wanted; but it will
nevertheless be not entirely useless to consider other possible
theories.

It would be quite conceivable to suppose that the youngest egg-cells,
which multiply by division, may undergo one ‘reducing division’ in
addition to the ordinary process. Of course this should occur once
only, for if repeated, the number of ancestral idioplasms in the
nucleus of the germ-cell would undergo a decrease greater than could be
afterwards compensated by the increase due to fertilization. Thus the
number of ancestral germ-plasms would continually decrease in the
course of generations,—a process which would necessarily end with their
complete reduction to a single kind, viz. to the paternal or the
maternal germ-plasm. But the occurrence of such a result is disproved
by the facts of heredity. Although such an early occurrence of the
‘reducing division’ would offer advantages in that nothing would be
lost, for both daughter-nuclei would become eggs, instead of one of
them being lost as a polar body, nevertheless I do not believe that it
really occurs: weighty reasons can be alleged against it.

Above all, the facts of parthenogenesis are against it. If the number
of ancestral germ-plasms received from the parents were reduced to half
in the ovary of the young animal, how then could parthenogenetic
development ever take place? It is true that we cannot at once assert
the impossibility of an early ‘reducing division’ on this account, for
as I have shown above, the power to develope parthenogenetically
depends upon the quantity of germ-plasm contained in the mature egg;
the necessary amount might be produced by growth, quite independently
of the number of different kinds of ancestral germ-plasms which form
its constituents. The size of a heap of grains may depend upon the
number of grains, and not upon the number of different kinds of grains.
But in another respect such a supposition would lead to an unthinkable
conclusion. In the first place, the number of ancestral germ-plasms in
the germ-cells would be diminished by one half in each new generation
arising by the parthenogenetic method; thus after ten generations only
1/1024 of the original number of ancestral germ-plasms would be present.

Now, it might be supposed that the ‘reducing division’ of the young
egg-cells was lost at the time when the parthenogenetic mode of
reproduction was assumed by a species; but this suggestion cannot hold,
because there are certain species in which the same eggs can develope
either sexually or parthenogenetically (e.g. the bee). It seems to me
that such cases distinctly point to the fact that the reduction in the
number of ancestral germ-plasms must take place immediately before the
commencement of embryonic development, or, in other words, at the time
of maturation of the egg. It is only decided at this time whether the
egg of the bee is to develope into an embryo by the parthenogenetic or
the sexual method; such decision being brought about, as was shown
above, by the fact that only one polar body is expelled in the first
case, while two are expelled in the second. But if we are obliged to
assume that reproduction by means of fertilization, necessarily implies
a reduction to one half of the number of ancestral germ-plasms
inherited from the parents,—the further conclusion is obvious, that the
second division of the egg-nucleus and the expulsion of the second
polar body represent such a reduction, and that this second division of
the egg-nucleus is unequal in the sense mentioned above, viz. one half
of the ancestral germ-plasms remains in the egg-nucleus, the original
number being subsequently restored by conjugation with a sperm-nucleus;
while the other half is expelled in the polar body and perishes.

I may add that observations, so far as they have extended to such
minute processes, do indeed prove that the number of loops is reduced
to one half. It has been already mentioned that, according to Carnoy,
such reduction occurs in _Ascaris megalocephala_, but the same author
also describes the process of the formation of polar bodies in a large
number of other _Nematodes_[263], and his descriptions show that the
process occurs in such a way that the number of ancestral germ-plasms
must be reduced by half. Sometimes half the number of primary loops
pass into the nucleus of the polar body, while the other half remains
in the egg. In other cases, as in _Ophiostomum mucronatum_, the primary
nuclear rods divide transversely,—a process which must produce the same
effect. It is true that these observations require confirmation, and
since, with unfavourable objects, the difficulties of observation are
extremely great, there may have been errors of detail; but I do not
think that there is any reason for doubting the accuracy of the
essential point. And this essential point is the fact that the number
of primary loops is divided into half by the formation of the polar
body.

But even if we could not admit that such a conclusion is securely
founded, it cannot be doubted that the formation of the second polar
body reduces to one half the quantity of the nucleus which would have
become the segmentation-nucleus in the parthenogenetic development of
the egg. This is a simple logical conclusion from the two following
facts: first, parthenogenetic eggs expel only _one_ polar body;
secondly, there are eggs (such as those of the bee) in which it is
absolutely certain that the same half of the nucleus—which is expelled
as the second polar body in the egg requiring fertilization—remains in
the egg when it is to develope parthenogenetically, and acts as half of
the segmentation-nucleus. But this proves that the expelled half of the
nucleus must consist of true germ-plasm, and thus a secure foundation
is laid for the assumption that the formation of the nucleus of the
second polar body must be considered as a ‘reducing division.’

I was long ago convinced that sexual reproduction must be connected
with a reduction in the number of ancestral germ-plasms to one half,
and that such reduction was repeated in each generation. When, in 1885,
I brought forward my theory of the continuity of the germ-plasm, I had
long before that time considered whether the formation and expulsion of
polar bodies must not be interpreted in this sense. But the two
divisions of the egg-nucleus caused me to hesitate. The two divisions
did not seem to admit of such an interpretation, for by it the quantity
of the nucleus is not divided into halves, but into quarters. But a
division of the number of ancestral germ-plasms into quarters would
have caused, as was shown above, a continuous decrease, leading to
their complete disappearance; and such a conclusion is contradicted by
the facts of heredity. For this reason I was led at that time to oppose
Strasburger’s view that the expulsion of the polar bodies means a
reduction of the quantity of nuclear substance by only half. My
objection to such a view was valid when I said that the quantity of
idioplasm contained in the egg-nucleus is not, as a matter of fact,
reduced to one half, but to one quarter, inasmuch as two successive
divisions take place. I may add that I had also considered whether the
two successive divisions might not possess an entirely different
meaning,—whether one of them led to the removal of ovogenetic
nucleoplasm, while the other resulted in a reduction in the number of
ancestral germ-plasms. But at that time there were no ascertained facts
which supported the supposition of such a difference, and I did not
wish to bring forward the idea, even as a suggestion, when there was no
secure foundation for it. The morphological aspects of the formation of
the first and second polar bodies are so extremely similar that such a
supposition might have been considered as a mere effort of the
imagination.

Hensen[264] also rejected the second part of the supposition that
reduction must take place in the number of the hereditary elements of
the egg, and that such reduction is caused by the expulsion of polar
bodies, because he believed it to be incompatible with the fact, which
had just been discovered, that polar bodies are formed by
parthenogenetic eggs. He concludes with these words: ‘If this striking
fact be confirmed, the hypothesis which assumes that the egg must be
divided into half before maturation, is refuted, and there only remains
the rather vague explanation that a process of purification must
precede the development of the embryo.’ Nevertheless Hensen is the only
writer who has hitherto taken into consideration the idea that sexual
reproduction causes a regularly occurring ‘diminution in the hereditary
elements of the egg.’

   III. THE FOREGOING CONSIDERATIONS APPLIED TO THE MALE GERM-CELLS.

If the result of the previous considerations be correct, and if the
number of ancestral germ-plasms contained in the nucleus of the
egg-cell destined for fertilization must be reduced by one half, there
can be no doubt that a similar reduction must also take place, at some
time and by some means, in the germ-plasms of the male germ-cells. This
must be so if we are correct in maintaining that the young germ-cells
of a new individual contain the same nuclear substance, the same
germ-plasm, which was contained in the fertilized egg-cell from which
the individual has been developed. The young germ-cells of the
offspring must contain this substance if my theory of the continuity of
the germ-plasm be well founded, for this theory supposes that, during
the development of a fertilized egg, the whole quantity of germ-plasm
does not pass through the various stages of ontogenetic development,
but that a small part remains unchanged, and at a later period forms
the germ-cells of the young organism, after having undergone an
increase in quantity. According to this supposition therefore the
germ-plasm of the parents must be found unchanged in the germ-cells of
the offspring. If this theory were false, if the germ-plasm of the
germ-cells were formed anew by the organism, perhaps from Darwin’s
‘gemmules’ which pour into the germ-cells from all sides, it would be
impossible to understand why it has not been long ago arranged that
each germ-cell should receive only half the number of the ancestral
gemmules present in the body of the parent. Hence the expulsion of the
second polar body—assuming the validity of my interpretation—is an
indirect proof of the soundness of the theory of the continuity of the
germ-plasm, when contrasted with the theory of pangenesis. If
furthermore, a kind of cyclical development of the idioplasm took
place, as supposed by Strasburger, and if its final ontogenetic stage
resulted in the re-appearance of the initial condition of the
germ-plasm, we should fail to understand how any of the ancestral
germ-plasms could be lost during such a course of development.

Whichever view, the latter or the theory of the continuity of the
germ-plasm, be correct, in either case the male germ-cells of the young
animal must contain the same germ-plasm as that which existed in the
fertilized maternal egg, that is to say, they must contain all the
ancestral germ-plasms of the father and the mother. Here therefore a
reduction must occur, for otherwise the number of ancestral germ-plasms
would be increased by one half at every fertilization. The egg-cell
would furnish 1/2, but the sperm-cell 2/2 of the total quantity of
germ-plasm present in the germ-cells of the parents. But there is no
reason for believing that the reduction of germ-plasm in the sperm-cell
must proceed in precisely the same way as in the egg-cell, viz. by the
expulsion of a polar body. On the contrary, the processes of
spermatogenesis are so remarkably different from those of ovogenesis
that we may expect to find that reduction is also brought about in a
different manner.

The egg-cell does not expel the superfluous ancestral germ-plasms until
the end of its development, and in a form which induces the destruction
of the separated portion. This is certainly remarkable, for germ-plasm
is a most important substance, and although it seems to be wasted in
the production of enormous quantities of sperm- and egg-cells, such
waste is only apparent, and is in reality the means which renders the
species capable of existence. It may perhaps be possible to prove that
in this case also the waste is only apparent. Such proof would be
forthcoming if it could be shown that the means by which reduction is
brought about in eggs is advantageous, and therefore also, _ceteris
paribus_, necessary. We see that everywhere, as far as our observation
extends, the useful is also the actual, unless indeed it is impossible
of attainment or can only be attained by the aid of processes which are
injurious to the species. And if it be asked why germ-plasm is wasted
in the maturation of egg-cells, the following may perhaps be a
satisfactory answer.

Let us suppose that the necessary reduction of the germ-plasm does not
take place by the separation of the second polar body, but that it
happens during the first division of the first primitive-germ-cell
which is found in the embryo, so that the two first egg-cells resulting
from this division would already contain only half the number of
ancestral germ-plasms from the father and the mother, contained in the
fertilized egg-cell. In this case the main object, the reduction of the
ancestral germ-plasms, would be gained by a single division, and all
the succeeding nuclear divisions, causing the multiplication of these
two first germ-cells, might take place by the ordinary form of nuclear
division, viz. ‘equal division.’ But perhaps nature not only cares for
this one main object alone, but also secures certain secondary
advantages at the same time. In the case which we have supposed the
egg-cells of the mature ovary would only contain two different
combinations of germ-plasm, which we may call combinations _A_ and _B_.
Even if millions of egg-cells were formed, every one of them would
contain either _A_ or _B_, and hence (at least as far as the female
pronucleus is concerned) only two kinds of individuals could arise from
such eggs—viz. offspring _A’_ and _B’_. All the offspring _A’_ would be
as similar to one another as identical twins, and the same would be
true of offspring _B’_.

But if the 100th instead of the 1st embryonic germ-cell entered upon
the ‘reducing division,’ a hundred cells would undergo this division at
the same time, and thus two hundred different combinations of ancestral
germ-plasm would arise, and two hundred different kinds of germ-cells
would be found in the mature ovary. A still greater number of different
combinations of hereditary tendencies would arise if the ‘reducing
division’ occurred still later; but undoubtedly the diversity in the
composition of the germ-plasm must be greatest of all when the
‘reducing division’ does not take place during the period in which the
germ-cells undergo multiplication, but at the end of the entire course
of ovarian development, and separately in each full-grown mature egg
ready for embryonic development. In such a case there will be as many
different combinations of ancestral germ-plasms as there are eggs, for,
as I have shown above, it is hardly conceivable that such a complex
body as the nuclear substance of the egg-cell—composed of innumerable
different units—would ever divide twice in precisely the same manner.
Every egg will therefore contain a somewhat different combination of
hereditary tendencies, and thus the offspring which arise from the
different germ-cells of the same mother can never be identical. Hence
by the late occurrence of the ‘reducing division’ the greatest possible
variability in the offspring is secured.

If my interpretation of the second polar body be accepted, it is
obvious that the late occurrence of the ‘reducing division’ is proved.
At the same time we receive an explanation of the advantage gained by
the postponement of the reduction of the germ-plasm until the end of
the ovarian development of the egg; because the greatest possible
number of individual variations in the offspring are produced in this
way.

If I am not mistaken, this argument lends additional support to the
idea which I have previously propounded,—that the most important duty
of sexual reproduction is to preserve and continually call forth
individual variability, the foundation upon which the transformation of
species is built[265].

But if it be asked whether the postponement of the ‘reducing division’
to the end of the ovarian development of the egg is inconsistent with
the preservation of the other half of the dividing nucleus, I should be
inclined to reply that a ‘reducing division’ of the mature egg,
resulting in the production of two eggs, was probably the phyletic
precursor of the present condition. I imagine that the division of the
mature egg-cell—although it is now so extremely unequal—was equal in
very remote times; but that for reasons of utility, connected with the
specialization of the eggs of animals, it gradually became more and
more unequal. It is now hardly possible to give in detail the various
reasons of utility which have brought about this condition, but it may
be assumed that the enormous size attained by many animal egg-cells has
been especially potent in producing the change.

A careful consideration of this last point seems to me to be demanded
by a comparison of the egg-cells with the male germ-cells. Just as the
female germ-cells of animals are distinguished by the attainment of a
large size, the male germ-cells are generally remarkable for their
minute proportions. In most cases it would be physiologically
impossible for a large egg-cell, rich in yolk, to attain double its
specific size in order to undergo division into two equal halves and
yet to remain of the characteristic size. Even without the additional
difficulties imposed by the necessity for such a division, all
means—such as cells used as food, or the passage of food from
follicular cells into the ovum, etc.—are employed in order to bring the
egg-cell to the greatest attainable size. Furthermore, the ‘reducing
division’ of the nucleus cannot take place before the egg has attained
its full size, because the ovogenetic nucleoplasm still controls the
egg-cell, and must be removed before the germ-plasm can regulate its
development. By arguments such as these I should attempt to render the
whole subject intelligible.

But the case is entirely different with the sperm-cells, which are
generally minute: here it is quite conceivable that a ‘reducing
division’ of the nuclei may take place by an equal division of the
sperm-cells, occurring towards the end of the period of their
formation; that is to say, in such a way that both products of division
remain sperm-cells, and neither of them perishes like the polar bodies.
But the other possibility also demands consideration, viz. that the
reducing division may occur at an earlier stage in the development of
sperm-cells. At all events, the arguments adduced above, which proved
that the consequence would be a want of variability in the egg-cells,
would not apply to an equal extent in the case of the male germ-cells.
Among the egg-cells it may be very important that each one should have
its special individual character, produced by a somewhat different
composition of its germ-plasm, inasmuch as a considerable proportion of
the eggs frequently developes, although this is never the case with all
of them. But the production of sperm-cells is in most animals so
enormous that only a very small percentage can be used for
fertilization. If, therefore, e. g. ten or a hundred spermatozoa
contained germ-plasm with exactly the same composition, so that, as far
as the paternal influence is concerned, ten or a hundred identical
individuals would result if they were all used in fertilization, such
an arrangement would be practically harmless, for only one spermatozoon
out of an immense number would be employed for this purpose. From this
point of view we might expect that the ‘reducing division’ of the
sperm-nucleus would not take place at the end of the development of the
sperm-cell, but at some earlier period. There is no necessary reason
for the assumption that this division must take place at the end of
development, and without some cause natural selection cannot operate.
It is, of course, conceivable that the causes of other events may also
involve the occurrence of this division at the end of development; but
we do not at present know of any such causes. I should not consider the
influence of the specific histogenetic nucleoplasm, i.e. the
spermatogenetic nucleoplasm, to be such a cause, because the
quantitative proportions are very different from those which obtain in
the formation of egg-cells, and because it is not inconceivable that
the small quantity of true germ-plasm which must be present in the
nuclei of the sperm-cells at every stage in their formation might enter
upon a ‘reducing division’ with the spermatogenetic nucleoplasm, even
when the latter preponderated.

As soon as we can recognize with certainty the forms of nuclear
division which are ‘reducing divisions,’ the question will be settled
as far as spermatogenesis is concerned. It has been already established
that various forms of nuclear division occur at different periods of
spermatogenesis. I make this assertion, not only from my own
observations, but also from observations which have been made and
insisted upon by others. Thus, van Beneden and Julin[266] stated in
1884 that direct and karyokinetic nuclear divisions alternate with each
other in the spermatogenesis of _Ascaris megalocephala_. Again,
Carnoy[267] distinctly states that the different cell-generations in
the same testis may not uncommonly exhibit considerable differences as
regards karyokinesis. ‘This may go so far that direct and indirect
division may proceed simultaneously.’ Platner[268], in his excellent
paper on karyokinesis in Lepidoptera, also points out that the
karyokinesis of the spermatocytes is essentially different from that of
the spermatogonia. According to his description, the latter form may be
very well interpreted as a ‘reducing division,’ for no equatorial plate
is formed, and the chromatin rods (or granules, as they are better
called in this case) remain from the first on both sides of the
equatorial plane, and finally unite at the opposite poles to form the
two daughter-nuclei. Furthermore, if Carnoy has correctly observed, the
form of karyokinesis which I have previously interpreted as a ‘reducing
division’ occurs in the sperm-mother-cells—a karyokinesis in which the
chromatin rods either do not divide longitudinally, or else divide in
this way after they have left the equatorial plate and are proceeding
towards the poles. Carnoy does not himself attach any special
importance to these observations, for he only considers them as proofs
that the longitudinal splitting of the loops may occur at various
periods in different species—either at the equator, or on the way
towards the poles, or even at the poles themselves. We cannot conclude
from the author’s statements whether this form of nuclear division only
occurs in a single cell-generation during spermatogenesis, as it must
do if it really represents a ‘reducing division.’ Until this point is
settled, we cannot decide with certainty whether the described form of
karyokinesis is to be considered as the ‘reducing division’ for which
we are seeking. Fresh investigations, undertaken from these points of
view, are necessary in order to settle the question. It would be
useless to seek further support for the theory by going into further
details, and by critically examining the numerous observations upon
spermatogenesis which have now been recorded.

I will only mention that among the various nuclei and other bodies in
different animals which have been considered by different observers as
the polar bodies of the sperm-cells, or the cells which form the
latter—in my opinion the paranucleus (‘Nebenkern’) of the ‘spermatides’
described by La Valette St. George[269] has the highest claim to be
considered as the homologue of a polar body. But I am inclined to
identify it with the first rather than the second polar body of the
egg-cells, and to regard it as the histogenetic part of the nucleoplasm
which has been expelled or rendered powerless by internal
transformations. There are two reasons which lead me to this
conclusion: first, as I have tried to show above, it is probable that
the ancestral germ-plasms are not removed by expulsion, but by means of
equal cell-division; secondly, my theory asserts that the histogenetic
nucleoplasm cannot be rendered powerless until the close of
histological differentiation.

The whole question of the details of the transformations undergone by
the nucleus of the male germ-cells is not ready for the expression of a
mature opinion. From the very numerous and mostly minute and careful
observations which have been hitherto recorded, we cannot conclude with
any degree of certainty when and how the ‘reducing division’ of the
nucleus takes place, nor can we decide upon the processes which signify
the purification of the germ-plasm from the merely histogenetic part of
the nucleoplasm. But perhaps it has not been without value as regards
future investigation that I have tried to apply to the male germ-cells
the views gained from our more certain knowledge of the corresponding
structures in the female, and thus to indicate the problems which now
chiefly demand solution.

          IV. THE FOREGOING CONSIDERATIONS APPLIED TO PLANTS.

It remains to briefly consider the case of plants. Obviously, the
‘reducing division’ of the germ-nuclei, if it takes place at all,
cannot be restricted to the germ-cells of animals. There must be a
corresponding process in plants, for sexual reproduction is essentially
the same in both kingdoms; and if fertilization must be preceded by the
expulsion of half the number of ancestral germ-plasms from the eggs of
animals, the same necessity must hold in the case of plants.

But whether the process always takes place in the form of polar bodies,
and not perhaps principally, or at any rate frequently, in the form of
equal cell-division, is another question. It is true that polar bodies
occur in numerous plants, as we chiefly know from Strasburger’s
researches[270]. Strasburger shows that cells are separated by division
from the germ-cells, and perish. But it seems to me doubtful whether we
must always regard their formation as the removal of half the number of
ancestral germ-plasms rather than the histogenetic nucleoplasm of the
germ-cell. It appears to me that histogenetic nucleoplasm must be
present in the highly differentiated vegetable germ-cells, especially
in the male cells, and also that it must be removed during the
maturation of the cell, if my idea of the histogenetic nucleoplasm be
accepted. It is very possible, as I have already mentioned, that there
may be quite indifferent germ-cells, viz. cells which are entirely
without specific histological structure, and in such cases histogenetic
nucleoplasm would be absent; and during the maturation of such
germ-cells no polar body would be formed for its removal. This view
accords with the fact that polar bodies are absent in many plants.
Furthermore, I am far from maintaining that in the cases where polar
bodies occur, they must have the above-mentioned significance. I only
wish to point out that the reduction assumed to be necessary for the
nucleus of the vegetable germ-cells is not necessarily to be sought for
at the close of their maturation, but perhaps even more frequently in
an equal division of the germ-cells during some period of their
development.

It also seems to me to be not impossible that a number of these
vegetative ‘polar bodies’ may have an entirely different significance,
viz. to perform some special function accessory to fertilization, as in
the so-called ‘ventral canal-cells’ of the higher cryptogams and
conifers. As we know that even the two polar bodies of the animal egg
are not identical—although externally they are extremely similar, and
although they arise in a precisely similar manner—I am even more
inclined than before to consider that the very various ‘polar bodies’
of plants possess very different meanings.

But I do not feel justified in criticizing in detail the results of
botanical investigation. I must leave the decision of such questions to
botanists, and I only desire to state distinctly that a ‘reducing
division’ of the nuclei of germ-cells must occur in plants as well as
in animals.

                V. CONCLUSIONS WITH REGARD TO HEREDITY.

The ideas developed in the preceding paragraphs lead to remarkable
conclusions with regard to the theory of heredity,—conclusions which do
not harmonize with the ideas on this subject which have been hitherto
received. For if every egg expels half the number of its ancestral
germ-plasms during maturation, the germ-cells of the same mother cannot
contain the same hereditary tendencies, unless of course we make the
supposition that corresponding ancestral germ-plasms are retained by
all eggs—a supposition which cannot be sustained. For when we consider
how numerous are the ancestral germ-plasms which must be contained in
each nucleus, and further how improbable it is that they are arranged
in precisely the same manner in all germ-cells, and finally how
incredible it is that the nuclear thread should always be divided in
exactly the same place to form corresponding loops or rods,—we are
driven to the conclusion that it is quite impossible for the ‘reducing
division’ of the nucleus to take place in an identical manner in all
the germ-cells of a single ovary, so that the same ancestral
germ-plasms would always be removed in the polar bodies. But if one
group of ancestral germ-plasms is expelled from one egg, and a
different group from another egg, it follows that no two eggs can be
exactly alike as regards their contained hereditary tendencies: they
must all differ. In many cases the differences will only be slight,
that is, when the eggs contain very similar combinations of ancestral
germ-plasms. Under other circumstances the differences will be very
great, viz. when the combinations of ancestral germ-plasms retained in
the egg are very different. I might here mention various other
considerations; but this would lead me too far from my subject, into
new theories of heredity. I hope to be able at some later period to
develope further the theoretical ideas which are merely indicated in
the present essay. I only wish to show that the consequences which
follow from my theory upon the second division of the egg-nucleus, and
the formation of the second polar body, are by no means opposed to the
facts of heredity, and even explain them better than has hitherto been
possible.

The fact that the children of the same parents are never entirely
identical could hitherto only be rendered intelligible by the vague
suggestion that the hereditary tendencies of the grandfather
predominate in one, and those of the grandmother in another, while the
tendencies of the great-grandfather predominate in a third, and so on.
Any further explanation as to why this should happen was entirely
wanting. Others even looked for an explanation to the different
influences of nutrition, to which it is perfectly true that the egg is
subjected in the ovary during its later development, according to its
position and immediate surroundings. I had myself referred to these
influences as a partial explanation[271], before I recognized clearly
how extremely feeble and powerless are the influences of nourishment,
as compared with hereditary tendencies. According to my theory, the
differences between the children of the same parents become
intelligible in a simple manner from the fact that each maternal
germ-cell (I shall speak of the paternal germ-cells later on) contains
a peculiar combination of ancestral germ-plasms, and thus also a
peculiar combination of hereditary tendencies. These latter by their
co-operation also produce a different result in each case, viz. the
offspring, which are characterized by more or less pronounced
individual peculiarities.

But the theory which explains individual differences by referring to
the inequality of germ-cells, may be proved with a high degree of
probability by an appeal to facts of an opposite kind, viz. by showing
that identity between offspring only occurs when they have arisen from
the same egg-cell. It is well known that occasionally some of the
children of the same parents appear to be almost exactly alike, but
such children are without exception twins, and there is every reason to
believe that they have been derived from the _same_ egg. In other
words, the two children are exactly alike because they have arisen from
the same egg-cell, which could of course only contain a single
combination of ancestral germ-plasms, and therefore of hereditary
tendencies[272]. The factors which by their co-operation controlled the
construction of the organism were the same, and consequently the
results were also the same. Twins derived from a single egg are
identical: this is a statement which, although not mathematically
proved, may be looked upon as nearly certain. But there are also twins
which do not possess this high degree of similarity, and these are even
far commoner than the others. The explanation is to be found in the
fact that the latter were derived from two egg-cells which were
fertilized at the same time. In most cases, indeed, each twin is
enclosed in its own embryonic membranes, while much less frequently
both twins are enclosed in the same membranes. In one point only the
proof is incomplete; for it has not yet been shown that identical twins
are always derived from a single egg, since such an origin, together
with a high degree of similarity, could only be established as
occurring together in a small proportion of the cases. We therefore see
that under conditions of nutriment which are as identical as possible,
_two_ egg-cells develope into unlike twins, _one_ into identical twins;
although we cannot yet affirm that the latter result invariably
follows. It is conceivable that the stimulus for the production of two
eggs from one may be afforded by the entrance of two spermatozoa, but
these latter, as was shown above, could hardly contain identical
hereditary tendencies, and thus two identical twins would not arise. It
appears indeed that some cases have been observed in which differences
have been exhibited by twins which were enclosed in the same embryonic
membranes; but nevertheless I believe that two spermatozoa are not
necessary to cause the formation of twins by a single egg. We know, it
is true, from the investigations of Fol[273], that multiple
impregnation produces the simultaneous beginning of several embryos in
the eggs of star-fishes. But several embryos and young animals are not
developed in this way, for embryonic development soon ceases, and the
egg dies.

The recent observations of Born[274] upon the eggs of the frog also
make it very probable that a double development is produced by the
entrance of two spermatozoa into the egg, but here also only
monstrosities, and not twins, were produced. On the other hand, it has
been shown that in birds twins may be produced from the same egg, and
there is no reason for the belief that their production is due to
multiple impregnation. But if it may be assumed that human twins, when
identical, have been derived from a single egg, it seems to me to be
extremely probable that fertilization was also effected by a single
sperm-cell. We cannot understand how such a high degree of similarity
could have been produced if two sperm-cells had been made use of, for
we are compelled to assume that two such cells would very rarely
contain identical germ-plasms.

It is most probable that the egg-nucleus coalesces with the nucleus of
a single spermatozoon, but the resulting segmentation-nucleus divides
together with the cell-body itself, without the occurrence of those
ontogenetic changes in the germ-plasm which normally take place. The
nucleoplasm of the two daughter-cells still remains in the condition of
germ-plasm, and its ontogenetic transformation begins afterwards—a
transformation which must of course proceed in the same way in both
cells, and must lead to the production of identical offspring. This is
at least a possible explanation which we may retain until it has been
either confirmed or disproved by fresh observations,—an explanation
which is moreover supported by the well-known process of budding in the
eggs of lower animals.

                          VI. RECAPITULATION.

To bring together shortly the results of this essay:—the fundamental
fact upon which everything else is founded is the fact that _two_ polar
bodies are expelled, as a preparation for embryonic development, from
all animal eggs which require fertilization, while only _one_ such body
is expelled from all parthenogenetic eggs.

This fact in the first place refutes every purely morphological
explanation of the process. If it were physiologically valueless, such
a phyletic reminiscence of the two successive divisions of the
egg-nucleus must have been also retained by the parthenogenetic egg.

In my opinion the expulsion of the first polar body implies the removal
of ovogenetic nucleoplasm when it has become superfluous after the
maturation of the egg has been completed. The expulsion of the second
polar body can only mean the removal of part of the germ-plasm itself,
a removal by which the number of ancestral germ-plasms is reduced to
one half. This reduction must also take place in the male germ-cells,
although we are not able to associate it confidently with any of the
histological processes of spermatogenesis which have been hitherto
observed.

Parthenogenesis takes place when the whole of the ancestral
germ-plasms, inherited from the parents, are retained in the nucleus of
the egg-cell. Development by fertilization makes it necessary that half
the number of these ancestral germ-plasms must be first expelled from
the egg, the original quantity being again restored by the addition of
the sperm-nucleus to the remaining half.

In both cases the beginning of embryogenesis depends upon the presence
of a certain, and in both cases equal, quantity of germ-plasm. This
certain quantity is produced by the addition of the sperm-nucleus to
the egg requiring fertilization, and the beginning of embryogenesis
immediately follows fertilization. The parthenogenetic egg contains
within itself the necessary quantity of germ-plasm, and the latter
enters upon active development as soon as the single polar body has
removed the ovogenetic nucleoplasm. The question which I have raised on
a previous occasion—‘When is the parthenogenetic egg capable of
development?’—now admits of the precise answer—‘Immediately after the
expulsion of the polar body.’

From the preceding facts and considerations the important conclusion
results that the germ-cells of any individual do not contain the same
hereditary tendencies, but are all different, in that no two of them
contain exactly the same combinations of hereditary tendencies. On this
fact the well-known differences between the children of the same
parents depend.

But the deeper meaning of this arrangement must doubtless be sought for
in the individual variability which is thus continuously kept up and is
always being forced into new combinations. Thus sexual reproduction is
to be explained as an arrangement which ensures an ever-varying supply
of individual differences.

------------------------------------------------------------------------

                        Footnotes for Essay VI.

Footnote 234:

  See Berichten der Naturforschenden Gesellschaft zu Freiburg i. B.,
  Band III. (1887) Heft I, ‘Ueber die Bildung der Richtungskörper bei
  thierischen Eiern,’ by August Weismann and C. Ischikawa.

Footnote 235:

  Vol. I. p. 60.

Footnote 236:

  The most recent example of this kind is afforded by the excellent
  work of O. Schultze, ‘Ueber die Reifung und Befruchtung des
  Amphibieneies,’ Zeitschr. f. wiss. Zool., Bd. XLV. 1887. Schultze has
  proved that two polar bodies are expelled from the egg of the Axolotl
  and of the frog, although all previous observers, including O.
  Hertwig, had been unable to find them. Thus the latter authority
  states as the result of an investigation specially directed towards
  this point, that the nucleus is transformed in a peculiar manner
  (‘Befruchtung des thierischen Eies,’ III. p. 81).

Footnote 237:

  O. Hertwig, ‘Beiträge zur Kenntniss der Bildung, Befruchtung, und
  Theilung des thierischen Eies,’ Morpholog. Jahrbuch, I, II, and III.
  1875-77.

Footnote 238:

  H. Fol, ‘Recherches sur la fécondation et le commencement de
  l’hénogénie chez divers animaux.’ Genève, Bâle, Lyon, 1879.

Footnote 239:

  Bütschli, ‘Entwicklungsgeschichtliche Beiträge,’ Zeitschr. f. wiss.
  Zool. Bd. XXIX. p. 237. 1877.

Footnote 240:

  C. S. Minot, ‘Account, etc.’ Proceedings Boston Soc. Nat. Hist., vol.
  xix. p. 165. 1877.

Footnote 241:

  F. M. Balfour, ‘Comparative Embryology.’

Footnote 242:

  Nägeli, ‘Mechanisch-physiologische Theorie der Abstammungslehre,’
  München und Leipzig, 1884.

Footnote 243:

  See the second and fourth Essays in the present volume.

Footnote 244:

  Hensen, ‘Die Grundlagen der Vererbung,’ Zeitschr. f. wiss.
  Landwirthschaft. Berlin, 1885, p. 749.

Footnote 245:

  O. Hertwig, ‘Lehrbuch der Entwicklungsgeschichte des Menschen und der
  Wirbelthiere.’ Jena, 1886.

Footnote 246:

  Bütschli, ‘Gedanken über die morphologische Bedeutung der sog.
  Richtungskörperchen,’ Biol. Centralblatt, Bd. VI. p. 5. 1884.

Footnote 247:

  This observation was first published as a note at the end of the
  fourth Essay in the present volume. See p. 249.

Footnote 248:

  Weismann, ‘Richtungskörper bei parthenogenetischen Eieren,’ Zool.
  Anzeiger, 1886, p. 570.

Footnote 249:

  Blochmann, ‘Ueber die Richtungskörper bei den Insekteneiern,’ Biolog.
  Centralblatt., April 15, 1887.

Footnote 250:

  F. Stuhlmann, ‘Die Reifung des Arthropodeneies nach Beobachtungen an
  Insekten, Spinnen, Myriapoden und Peripatus,’ Berichte der
  naturforschenden Gesellschaft zu Freiburg i. Br., Bd. I. p. 101.

Footnote 251:

  In the summer-eggs of Rotifera I have, together with Mr. Ischikawa,
  observed one polar body, and we were able to establish for certain
  that a second is not formed. The nuclear spindle had already been
  observed by Tessin, and Billet had noticed polar bodies in
  _Philodina_, but without attaching any importance to their number.
  These latter observations were not conclusive proofs of the formation
  of polar bodies in parthenogenetic eggs, so long as it was not known
  whether the summer-eggs of Rotifera may develope parthenogenetically,
  or whether they can only develope in this way. Knowing now that
  parthenogenetic eggs expel only one polar body, we may perhaps be
  permitted to draw the conclusion that the summer-egg of a Rotifer
  (_Lacinularia_) which expelled only one polar body must have been a
  parthenogenetic egg. But I may add that we have also succeeded in
  directly proving the occurrence of parthenogenesis in Rotifera, as
  will be described in detail in another paper.

Footnote 252:

  See Essay IV, Part III. p. 225.

Footnote 253:

  E. Bessels, ‘Die Landois’sche Theorie, widerlegt durch das
  Experiment.’ Zeitschr. f. wiss. Zool. Bd. XVIII. p. 124. 1868.

Footnote 254:

  l. c., p. 110.

Footnote 255:

  Strasburger, ‘Neue Untersuchungen über den Befruchtungsvorgang bei
  den Phanerogamen als Grundlage einer Theorie der Zeugung.’ Jena, 1884.

Footnote 256:

  Wilhelm Roux, ‘Ueber die Bedeutung der Kerntheilungsfiguren.’
  Leipzig, 1884.

Footnote 257:

  E. van Beneden, ‘Recherches sur la maturation de l’œuf, la
  fécondation et la division cellulaire.’ Gand et Leipzig, Paris, 1883.

Footnote 258:

  J. B. Carnoy, ‘La Cytodiérèse de l’œuf, la vésicule germinative et
  les globules polaires de l’Ascaris megalocephala.’ Louvain, Gand,
  Lierre, 1886.

Footnote 259:

  See p. 364.

Footnote 260:

  Wilhelm Roux, ‘Beiträge zur Entwicklungsmechanik des Embryo,’ No. 3,
  Breslauer ärztliche Zeitschrift, 1885, p. 45.

Footnote 261:

  Carnoy, ‘La Cytodiérèse chez les Arthropodes.’ Louvain, Gand, Lierre,
  1885.

Footnote 262:

  Flemming, ‘Neue Beiträge zur Kenntniss der Zelle.’ Arch. f. mikr.
  Anat. Bd. XXIX, 1887.

Footnote 263:

  Carnoy, ‘La Cytodiérèse de l’œuf; la vésicule germinative et les
  globules polaires chez quelques Nématodes.’ Louvain, Gand, Lierre.
  1886.

Footnote 264:

  Hensen, ‘Die Grundlagen der Vererbung nach dem gegenwärtigen
  Wissenskreis,’ Zeitschr. f. wissenschaftl. Landwirthschaft, Berlin,
  1885, p. 731.

Footnote 265:

  See the preceding Essay on ‘The Significance of Sexual Reproduction
  in the theory of Natural Selection.’

Footnote 266:

  E. van Beneden and Julin, ‘La Spermatogénèse chez l’Ascaride
  mégalocéphale.’ Brussels, 1884.

Footnote 267:

  Carnoy, ‘La Cytodiérèse chez les Arthropodes.’

Footnote 268:

  Gustav Platner, ‘Die Karyokinese bei den Lepidopteren als Grundlage
  für eine Theorie der Zelltheilung.’ Internation. Monatsschrift f.
  Anatomie und Histologie, Bd. III. Heft 10. Leipzig, 1886.

Footnote 269:

  La Valette St. George, ‘Ueber die Genese der Samenkörper.’ Fünfte
  Mittheilung. Die Spermatogenese bei den Säugethieren und dem
  Menschen,’ Archiv f. mikrosk. Anat. Bd. XV. 1878.

Footnote 270:

  Weismann, ‘Studien zur Descendenztheorie,’ ii. p. 306, Leipzig, 1876,
  translated by Meldola; see ‘Studies in the Theory of Descent,’ p. 680.

Footnote 271:

  l. c., p. 92.

Footnote 272:

  [The similar conclusion that identical ova lead to the appearance of
  identical individuals was drawn from the same data by Francis Galton
  in 1875. See ‘The history of the Twins, as a criterion of the
  relative powers of Nature and Nurture,’ by Francis Galton, F.R.S.,
  Journal of the Anthropological Institute, 1875, p. 391; also by the
  same author, ‘Short Notes on Heredity, etc. in Twins,’ in the same
  Journal, 1875, p. 325.

  The author investigated about eighty cases of close similarity
  between twins, and was able to obtain instructive details in
  thirty-five of these. Of the latter there were no less than seven
  cases ‘in which both twins suffered from some special ailment or had
  some exceptional peculiarity;’ in nine cases it appeared that ‘both
  twins are apt to sicken at the same time;’ in eleven cases there was
  evidence for a remarkable association of ideas; in sixteen cases the
  tastes and dispositions were described as closely similar. These
  points of identity are given in addition to the more superficial
  indications presented by the failure of strangers or even parents to
  distinguish between the twins. A very interesting part of the
  investigation was concerned with the after-lives of the thirty-five
  twins. ‘In some cases the resemblance of body and mind had continued
  unaltered up to old age, notwithstanding very different conditions of
  life,’ in the other cases ‘the parents ascribed such dissimilarity as
  there was, wholly, or almost wholly, to some form of illness.’

  The conclusions of the author are as follows: ‘Twins who closely
  resembled each other in childhood and early youth, and were reared
  under not very dissimilar conditions, either grow unlike through the
  development of natural characteristics which had lain dormant at
  first, or else they continue their lives, keeping time like two
  watches, hardly to be thrown out of accord except by some physical
  jar. Nature is far stronger than nurture within the limited range
  that I have been careful to assign to the latter.’ And again, ‘where
  the maladies of twins are continually alike, the clocks of their two
  lives move regularly on, and at the same rate, governed by their
  internal mechanism. Necessitarians may derive new arguments from the
  life histories of twins.’

  The above facts and conclusions held for twins of the same sex, of
  which at any rate the majority are shown by Kleinwächter’s
  observations to have been enclosed in the same embryonic membranes,
  and therefore presumably to have been derived from a single ovum; but
  in rarer cases the twins, although also invariably of the same sex,
  were marked by remarkable differences, greater than those which
  usually distinguish children of the same family. Mr. Galton met with
  twenty of these cases. In such twins the conditions of training, etc.
  had been as similar as possible, so that the evidence of the power of
  nature over nurture is strongly confirmed. Mr. Galton writes, ‘I have
  not a single case in which my correspondents speak of originally
  dissimilar characters having become assimilated through identity of
  nurture. The impression that all this evidence leaves on the mind is
  one of wonder whether nurture can do anything at all, beyond giving
  instruction and professional training.’

  The fact that twins produced from a single ovum seem to be invariably
  of the same sex is in itself extremely interesting, for it proves
  that the sex of the individual is predetermined in the fertilized
  ovum.—E. B. P.]

Footnote 273:

  Fol, Recherches sur la fécondation et le commencement de l’hénogénie:
  Genève, Bâle, Lyon. 1879.

Footnote 274:

  Born, ‘Ueber Doppelbildungen beim Frosch und deren Entstehung.’
  Breslauer ärztl. Zeitschrift, 1882.

------------------------------------------------------------------------






                                  VII.

                    ON THE SUPPOSED BOTANICAL PROOFS

                                 OF THE

                  TRANSMISSION OF ACQUIRED CHARACTERS.

                                 1888.

   From ‘Biologisches Centralblatt,’ Bd. VIII. Nr. 3 and 4, pages 65
                          and 97: April 1888.




------------------------------------------------------------------------

                                  VII.

                    ON THE SUPPOSED BOTANICAL PROOFS
                                 OF THE
                  TRANSMISSION OF ACQUIRED CHARACTERS.


In a lecture on heredity, delivered in 1883[275], I first brought
forward the opinion that acquired characters cannot be transmitted; and
I then stated that there are no proofs of such transmission, that its
occurrence is theoretically improbable, and that we must attempt to
explain the transformation of species without its aid. Since that time
many biologists have expressed their opinions upon the subject, some of
them agreeing with me, while others have taken the opposite side. It is
unnecessary to allude to those who have attacked my opinions without
first understanding the real point in dispute, which turns upon the
true meaning of the phrase ‘acquired character.’ I think it is now
generally admitted that a very important problem is involved in this
question, the solution of which will contribute in a decisive manner
towards the formation of ideas as to the causes which have produced the
transformation of species. For if acquired characters cannot be
transmitted, the Lamarckian theory completely collapses, and we must
entirely abandon the principle by which alone Lamarck sought to explain
the transformation of species,—a principle of which the application has
been greatly restricted by Darwin in the discovery of natural
selection, but which was still to a large extent retained by him. Even
the apparently powerful factors in transformation—the use and disuse of
organs, the results of practice or neglect—cannot now be regarded as
possessing any direct transforming influence upon a species. And the
same is true of all the other direct influences, such as nutrition,
light, moisture, and that combination of different influences which we
call climate. All these, with use and disuse, may perhaps produce great
effects upon the body (_soma_) of the individual, but cannot produce
any effect in the transformation of the species, simply because they
can never reach the germ-cells from which the succeeding generation
arises. But if—as it seems to me—the facts of the case compel us to
reject the assumption of the transmission of acquired characters, there
only remains one principle by which we can explain the transformation
of species—the direct alteration of the germ-plasm, however we may
imagine that such alterations have been produced and combined to form
useful modifications of the body.

The difficulty of understanding these processes of transformation is by
no means lightened by abandoning the Lamarckian theory. The difficulty
in fact becomes much greater, for we are now compelled to seek a
different explanation of many phenomena which were previously believed
to be understood. But this can hardly be regarded as a reason for not
accepting the view: for we are in want of a correct explanation rather
than one which is easy and convenient. We seek truth, and when we
recognize that our path is leading in a wrong direction, we must leave
it and take another road even if it presents more difficulties.

My theory rests, on the one hand, upon certain theoretical
considerations which will be mentioned below, and which I have
attempted to develope in previous papers[276]. On the other hand, it
rests upon the want of any actual proof of the transmission of acquired
characters. My theory might be disproved in two ways,—either by
actually proving that acquired characters are transmitted, or by
showing that certain classes of phenomena admit of absolutely no
explanation unless such characters can be transmitted. It will be
admitted, however, that we must be very cautious in accepting proofs of
this latter kind, for the impossibility of explaining a given
phenomenon may be merely temporary, and may disappear with the progress
of science. No one could have explained the useful adaptations so
common in animals and plants, before the light of the theory of natural
selection had fallen on these phenomena; at that time we should have
been far from right if we had assumed that organisms possess a power
which causes them to respond to external influences by useful
modifications, a power unknown elsewhere, entirely unproved and only
supported by the fact that at that time it did not seem possible to
explain the phenomena in any other way.

Although my theory has not been disproved, I will nevertheless attempt
to bring into further accordance with it certain phenomena which seem
at first sight to oppose it. I first began to take this course in my
paper ‘On Heredity[277].’ In that paper I attempted to show how the
fact that disused organs become rudimentary may be readily explained
without assuming the transmission of acquired characters; and also that
the origin of instincts may in all cases be referred to the process of
natural selection[278], although many observers had followed Darwin in
explaining them as inherited habits,—a view which becomes untenable if
the habits adopted and practised in a single life cannot be transmitted.

Other phenomena which appeared to present difficulties were also
considered and brought into accordance with the theory, and I think
that I have been successful in showing that adequate and simple
explanations may be given.

There certainly remain many phenomena which seem to be opposed to my
theory and for which a new explanation must be found. Thus
Romanes[279], following Herbert Spencer[280], has recently pointed to
the phenomena of correlation as a proof of the transmission of acquired
characters; but, at no distant time, I hope to be able to consider this
objection, and to show that the apparent support given to the old idea
is in reality insecure and breaks down as soon as it is critically
examined. I believe that I shall be able to prove that correlation
cannot be used as the indirect proof of an hypothesis, of which all
direct evidence is still completely wanting. It must not be forgotten
that the _onus probandi_ rests with my opponents: they defend the
assertion that acquired characters can be transmitted, and they ought
therefore to bring forward actual proofs; for the mere fact that the
assertion has been hitherto accepted as a matter of course by almost
everyone, and has only been doubted by a very few (such as His, du
Bois-Reymond, and Pflüger), cannot be taken as any proof of its
validity. Not a single fact hitherto brought forward can be accepted as
a proof of the assumption. Such proofs ought to be found: facts ought
to be discovered which can only be understood with the aid of this
hypothesis. If, for instance, it could be shown that artificial
mutilation spontaneously re-appears in the offspring with sufficient
frequency to exclude all possibilities of chance, then such proof would
be forthcoming. The transmission of mutilations has been frequently
asserted, and has been even recently again brought forward, but all the
supposed instances have broken down when carefully examined. I think I
may here safely omit all further reference to the proofs dependent upon
transmitted mutilations, especially as Döderlein[281] has already, in
the most convincing manner, disposed of the argument derived from the
tailless cats which were so triumphantly exhibited at the last meeting
of the Association of German Naturalists[282].

I now come to the real subject of this paper—the supposed botanical
proofs of the transmission of acquired changes. The botanist Detmer has
recently brought forward certain phenomena in vegetable
physiology[283], as a support for the transmission of such changes, and
although I do not believe that they will bear this interpretation, the
discussion of them may perhaps be useful. I am even inclined to think
that these and a few other phenomena in vegetable physiology, upon
which I shall also touch, are very likely to throw new light upon the
whole question which has been so frequently misunderstood. I should
have preferred to leave this discussion to a botanist, but I do not
know whether my views will meet with any support from the followers of
this subject, and I must therefore attempt the discussion myself. And
perhaps it is of some assistance in clearing up the question, for one
who is not accustomed to the usual botanical views, and is more
conversant with other classes of biological knowledge, to consider the
facts brought to light by modern botany, from a general point of view.
Of course I shall not attempt to question the validity of the
observations, nor even the accuracy with which the facts have been
interpreted. I shall only deal with the conclusions which may be drawn
from the facts, and I do not think that it is absolutely necessary that
such criticism should be made by a botanist. Questions of general
biological significance such as that of heredity cannot be entirely
solved within the single domain of either zoological or botanical
facts. Both botanists and zoologists must give due weight to the facts
of the province which is not their own, and must see whether the views
which they have chiefly gained in the one province can be applied to
the other, or whether phenomena occur in the latter which are in
opposition to their previously formed views and which cause them to be
abandoned or modified.

Detmer begins by bringing forward certain facts which prove, as he
believes, that rather important changes in the organism can be directly
produced by external influences. He is of opinion that I under-estimate
the weight of these influences, and that I make light of the changes
which may thus arise in a single individual life. But obviously, it is
of no importance for the question of the transmission of acquired
characters, whether the changes directly produced by external
influences upon the _soma_ of an individual are greater or smaller: the
only question is whether they can be transmitted. If they can be
transmitted, the smallest changes might be increased by summation in
the course of generations, into characters of the highest degree of
importance. It is in this way that Lamarck and Darwin have supposed
that an organism is transformed by external influences. It is therefore
interesting to see what Detmer considers to be a change which has been
directly effected. We can in this way gain a very distinct appreciation
of the difference in views which is caused by the different spheres of
experience which belong to botany and zoology. It will be useful to
gain a clear idea of the differences which are thus caused.

Detmer first alludes to the dorso-ventral structure of the shoots of
_Thuja occidentalis_, chiefly shown in the fact that the upper sides of
these shoots contain the green palisade cells, while the under sides
which are turned away from the light possess green spheroidal
(isodiametric) cells. If the branches of _Thuja_ are turned upside down
and fixed in this position before the production of new shoots, it is
found that the anatomical structure of the latter, when developed, is
reversed. The side of the shoot which was destined to become the under
side, but which was artificially compelled to become the upper side,
assumes the structure of the upper side and developes the
characteristic palisade parenchyma; and on the other hand, the under
side which was intended to become the upper side developes the spongy
parenchyma which is characteristic of the under side. From these facts
Detmer concludes that the dorso-ventral structure of the shoots of
_Thuja_ has resulted from the continual operation of an external force,
and that the light must be considered as the cause of the structural
change.

But such a conclusion obviously depends upon a confusion of ideas. No
one will doubt that the light was the stimulus which led to the
reversal of the structures in the shoot, but this is a very different
thing from maintaining that it was the cause which conferred upon the
_Thuja_-shoot the power of producing palisade and spongy parenchyma.
When a phenomenon only occurs under certain conditions, it does not
follow that these conditions are the cause of the phenomenon. A certain
temperature is necessary for the development of a bird in the egg, but
surely no one will maintain that the temperature is the cause of the
capacity for such development. It is obvious that the egg has acquired
the power of producing a bird chiefly as the result of a long phyletic
course of development which has led to such a chemical and physical
structure in the egg and the fertilizing sperm-cell, that after their
union and development, a bird, and only a bird of a particular species,
must be produced. But of course certain conditions must be fulfilled in
order that such development may take place; and a definite temperature
is one of these conditions of development. Thus we may briefly say that
the physical nature of the egg is the cause of its development into a
bird, and we may similarly maintain that the physical nature of a
_Thuja_-shoot, and not the influence of light, is the cause of the
development of tissues which are characteristic of the species. In the
development of such a shoot the light plays precisely the same part
which is played by temperature in the development of a bird: it is one
of the conditions of development.

There is nevertheless a difference between these two cases in that the
_Thuja_-shoot possesses the possibility of development in two different
ways instead of only one. The upper side of the shoot can assume the
structure of the under side and _vice versa_, and this structural
reversal depends upon the way in which the light is thrown upon the
shoot. But even if the light causes the structural reversal, does this
justify us in assuming that the structure itself is also the direct
consequence of the influence of light? I see no reason for rejecting
the supposition that the physical nature of part of a plant may be of
such a kind that this or that structure may be produced according as
this or that condition of development prevails. Thus with stronger
light the structure of the upper side of the shoot developes; with
weaker light, the structure of the under side. But this physical nature
of the _Thuja_-bud depends, like that of a bird’s egg, upon its
phyletic history, as we must assume to be the case with the germs
producing all individual developments. It is therefore quite impossible
to interpret the reversal of the structure in the _Thuja_-shoot as the
result of modification produced by the direct influence of external
conditions. It is an instance of double adaptation—one of those cases
in which the specific nature of a germ, an organism, or a part of an
organism, possesses such a constitution that it reacts differently
under the incidence of different stimuli. An entirely analogous example
of reversal occurs in the climbing shoots of the Ivy, and is described
in Sachs’ lectures on the physiology of plants. Such shoots produce
leaves only on the side directed towards the light, and roots (which
are made use of in climbing) only upon the opposite side. If however
the position of the plant be altered so that the root-bearing side is
turned towards the light, while the leafy side is shaded, a reversal
occurs, so that from that time the former only produces leaves, and the
latter nothing but roots. In other words, the Ivy-shoot reacts under
strong light with the production of leaves and under weak light with
the production of roots, just as litmus-paper becomes red with an acid
and blue with an alkali. The physical nature of the Ivy-shoot was
present before the production of either structure, and was no more due
to the action of light itself, than the physical nature of litmus-paper
is due to an acid or an alkali. But this is quite consistent with the
possession of a physical nature which reacts differently under the two
different conditions afforded by light and shade.

No one would think of bringing forward the changes in the colour of the
green frog (_Hyla_) as a proof of the power of direct influences in
causing structural modifications in the animal body. The frog is light
green when it is resting upon green leaves, but it becomes dark brown
or nearly black when transferred to dark surroundings. This is an
obvious instance of adaptation, for the changes in the colour of the
frog depend upon a complex reflex mechanism. The changes in the shape
of the chromatophores of the skin are not produced by the direct
influence of the different rays of light upon the body-surface, but in
consequence of the action of these rays upon the retina. Blind frogs do
not react under the changes of light. Hence it is impossible that any
one can maintain that the skin of the frog has gained its green colour
as the direct result of the green light reflected from its usual
surroundings. It must be admitted that in this and in all similar
cases, there is only one possible explanation, viz. an appeal to the
operation of natural selection. It may be objected that we are not here
dealing, as in the _Thuja_ and Ivy, with changes in the course of
ontogenetic development following upon the occurrence of this or that
external condition, but only with the different reactions of a mature
organism. But nevertheless, cases of the former kind appear to be also
present in the animal kingdom.

Thus the very careful and extensive investigations of Poulton[284] upon
the colours of certain caterpillars have distinctly shown that some
species possess the possibility of development in two directions, and
that the actual direction taken by the individual is decided by the
influence of external conditions. Poulton surrounded certain larvae of
Geometrae with an abundance of dark branches, in addition to the leaves
upon which they fed. When such conditions prevailed from the beginning
of larval life, the caterpillars as they developed, gradually assumed
the dark colour of the twigs and branches upon which they rested. When
other larvae of the same species (and in many experiments hatched from
the same batch of eggs) were similarly exposed to the green leaves of
the same food-plant, they did not indeed become bright green like the
leaves, but were invariably of a much lighter colour than the other
larvae, while many of them gained a brownish-green tint. The larvae of
_Smerinthus ocellatus_[285] also possess the power of assuming
different shades of green and of thus approaching, to some extent, the
green of the plant upon which they happen to live. It is quite
impossible to explain the phyletic development of the green colour of
these and other caterpillars as due to the direct action upon the skin
of the green light reflected from the leaves upon which they sit. The
impossibility of such an effect was pointed out long ago by Darwin, and
also followed from my own investigations. Here, as in the other cases,
the only possible solution is afforded by natural selection. The colour
of the caterpillars has become gradually more and more perfectly
adapted to the colour of the leaves,—and often to the particular side
of the leaves upon which these animals rest,—not by the direct effect
of reflected light, but by the selection of those individuals which
were best protected. Poulton’s experiments quoted above prove that
certain species which occur upon different plants with different
colours (or even in some cases upon the differently coloured parts of
the same plant), present us with a further complication in the process
of adaptation, inasmuch as each individual has acquired the power of
assuming a lighter or darker colour[286]. The light which falls upon a
single individual caterpillar during the course of its growth
determines whether the lighter or darker colour shall be developed.
Here therefore we have a case exactly parallel to that of the
_Thuja_-shoot in which the palisade or spongy parenchyma is developed
according to the position in which the shoot is fixed.

As far as it is possible in the present condition of our knowledge to
offer any opinion upon the origin of sex in bisexual animals, it may be
suggested that this problem is also capable of an essentially similar
solution. Each germ-cell may possess the possibility of developing in
either of two directions, the one resulting in a male individual, and
the other resulting in a female, while the decision as to which of the
two possible alternatives is actually taken may rest with the external
conditions. We must, however, include among the external circumstances
everything which is not germ-plasm. Moreover, this explanation is by no
means certain, and I only mention it as an instance which, if we assume
it to be correct, further illustrates my views upon the phenomena
presented by the _Thuja_-shoot.

The two other facts brought forward by Detmer as proofs of the
transforming power of external influences can be explained in precisely
the same manner. These instances are—the fact that _Tropaeolum_ when
grown in moist air produces leaves with anatomical characters different
from those produced when the plant is grown in dry air; and the
differences in the structure of the leaves of many plants, according as
they have been grown in the sun or shade respectively. Such differences
do not by any means afford proof of the direct production of structural
changes by means of external influences. How would such an explanation
be consistent with the fact that the leaves are, in all these cases,
changed in a highly purposeful manner? Or is it assumed that these
organs were so constituted from the beginning, that they are compelled
to respond to external conditions by the production of useful changes?
Any one who made such an assertion nowadays, or who even thought of
such a thing as a possibility, would prove that he is entirely ignorant
of the facts of organic nature, and that he has no claim to be heard
upon the question of the transformation of species. The very first
necessity in any scientific question is to gain acquaintance with that
which has been thought and said upon the subject. And it has been
frequently shown that whole groups of useful characters cannot by any
possibility have been produced by the direct action of external
influences. If a caterpillar, which hides itself by day in the crevices
of the bark, possesses the same colour as the latter, while other
caterpillars which rest on leaves are of a green colour, these facts
cannot be explained as the results of the direct influence of the bark
and leaves. And it would be even less possible to explain upon the same
principle all the details of marking and colour by which these animals
gain still further protection. If the upper side of the upper wings of
certain moths is grey like the stone on which they rest by day, while
in butterflies the under side of both wings which are exposed during
rest, exhibits analogous protective colours, these facts cannot be due
to the direct influence of the surroundings which are resembled, but,
if they have arisen in any natural manner, they must have been
indirectly produced by the surroundings. One may reasonably complain
when compelled to repeat again and again these elements of knowledge
and of thought upon the causes of transformation!

Any one who remembers these things, and is aware of the countless
number of purposeful characters which cannot possibly depend upon such
direct influences, will be very cautious in yielding to any single
instance which at first sight appears to be the direct consequence of
external conditions. If Detmer had been thus cautious he would hardly
have written the following sentence as a _résumé_ of the physiological
experiments on plants which have been already alluded to: ‘In certain
cases it is possible, as we have seen, to artificially modify the
anatomical structure of certain parts of plants. In such cases the
relation between the structure and the external influences is
undoubtedly clear: the latter act as the cause; the anatomical
structure of the members of the plant is the consequence of this
cause.’ A little more logic would have prevented the author from
expressing such an opinion, for, as has been already shown, it is
founded on a confusion between the true cause of a phenomenon and one
of the conditions which are necessary for its production. We might as
well consider the phenomena of _geotropism_, _hydrotropism_, and
_heliotropism_—which have been established, and investigated in such a
brilliant way by modern vegetable physiologists—as the direct results
of the attraction of the earth, of water, and of light; and it is not
improbable that some botanists are even inclined to make this
assumption. And yet it is perfectly easy to show that this cannot be
the case. By geotropism we mean the power possessed by the parts of a
plant of growing along lines which make certain angles with the
direction of the earth’s attraction. For example, the chief root grows
parallel with the earth’s attraction, viz. towards the centre of the
earth, and it is described as positively geotropic: conversely the main
shoot grows along the same line but in an opposite direction, and it is
negatively geotropic. But geotropism is not a primitive attribute of
the plant, and it is even now absent from those plants which, like many
Algae, have no definite position. Geotropism cannot have arisen before
plants first became fixed in the earth. If any one were to assume that
the direct influence of gravity, continuous through countless
generations, had at length conferred upon the root the power of growing
in a geotropic direction, how would it be possible to explain the fact
that the shoot which has been under precisely the same influence has
acquired the power of growing in an exactly opposite direction? The
characteristic differences between root and shoot cannot have appeared
until the plant became fixed in the ground, and how can we imagine that
the same influence of gravity has since that time directly produced the
two antagonistic results of positive and negative geotropism, in two
structures, which were originally and essentially similar? It should
also be remembered that it is only the main root which exhibits true
positive geotropism. The lateral roots form angles with the main root,
and do not therefore grow towards the earth’s centre; and the same is
true of the lateral shoots which grow obliquely, and not
perpendicularly upwards, like the main shoot. Moreover the angles which
the lateral roots make with the main root, and the lateral shoots with
the main shoot, are quite different in different species. How is it
possible that all these different modes of reaction witnessed in the
different parts of plants can be the direct results of one and the same
external force? It is quite obvious that these are all cases of
adaptation. The main root has not acquired the power of growing
perpendicularly downwards under the stimulus of gravity, because this
force has acted upon it for numberless generations, but because such a
direction for such a part was the most useful to the plant. Hence
natural selection has conferred upon the root the power of reacting
under the stimulus of gravity by growing in a direction parallel to
this force. For the main shoot, the opposite reaction was the most
useful and has been established by natural selection, while still
another reaction has been similarly established for the lateral roots
and another for the lateral shoots.

Each part of a plant has received its special mode of reacting under
the stimulus of gravity because it was useful for the whole plant,
inasmuch as the position of its different parts relatively to one
another and to the soil became thus fixed and regulated. These modes of
reaction have become different in different species, because the
conditions of life peculiar to each require special arrangements.

The same argument also holds with regard to heliotropism. The power of
growing towards the light possessed by green shoots cannot be a
primitive character of the plant: it must have arisen secondarily. If
it were an essential and original character it could not be reversed in
certain parts of the plant; but, the roots are negatively heliotropic,
for they grow away from the light. There are also shoots, such as the
climbing shoots of Ivy, which are similarly negatively heliotropic.
Whenever the heliotropic power is thus reversed in shoots, the change
is of a useful kind. Thus the shoots of the Ivy gain the power of
clinging closely to a perpendicular wall or to some horizontal
plane[287]. In this case, however, it is only the shoot which is
negatively heliotropic, its leaves turn towards the light; and the same
is true of the flower-bearing shoots which do not climb. All these are
clearly adaptations and not the results of direct influence. The light
only provides the stimulus which calls forth the characteristic
reaction from each part of the plant, but the cause of each peculiar
reaction lies in the specific nature of the part itself which has not
been produced by light, but as we believe by processes of natural
selection. If this explanation does not account for the facts we may as
well abandon all attempts at understanding the useful arrangements in
organisms.

Sachs has used the term _anisotropism_ to express the fact that the
various organs of a plant assume the most diverse directions of growth
under the influence of the same forces. He also states that
anisotropism is one of the most general characteristics of vegetable
organization, and that it is quite impossible to form any idea as to
how plants would appear or how they could live if their different
organs were not anisotropic. Since anisotropism is nothing more than
the expression of different kinds of susceptibility to the action of
gravity, light, &c., it is obvious that the configuration of the plant
is to be traced to such specific susceptibilities.

Now these specific susceptibilities cannot have been produced by the
direct effect of the various external influences (as was shown above),
and the only other possible explanation is to recognise them as
adaptations, and to admit that they have arisen by the operation of
natural selection upon the general variability of plant organization.

Simple as these conclusions are, I have failed to meet with them in any
of the writings of botanists, and they may perhaps be of use in helping
to shake the vaguely-felt opinion that the characters of plants are to
be chiefly referred to the direct action of external influences.

At all events it cannot be maintained that the phenomena of
anisotropism support the opinion mentioned above; and the mere
assertion that it is highly probable that hereditary characters arise
as the result of external influences, is no more than the expression of
an unfounded individual opinion. It is remarkable that Detmer should
make such an assertion as the outcome of his discussion of the reversed
_Thuja_-shoot, &c., for even if we admit that the dorso-ventral
structure of the shoot is—as Detmer believes—the direct and primary
effect of the action of light, the experiment with the reversed shoot
would prove that no part of this effect has become hereditary. Although
the upper side of the shoot has produced the palisade parenchyma under
the influence of light for thousands of generations, there is
nevertheless no tendency towards the establishment of any hereditary
effect, for as soon as the upper side of the growing shoot is
artificially transformed into the under side, its normal structure is
at once abandoned. Hence so far from lending any support to the
assumption that acquired characters can be transmitted, Detmer’s
experiment rather tends to disprove this opinion.

I think I have sufficiently shown that Detmer’s reproach—that I have
under-estimated the effects of external influences upon an organism—may
be fairly directed against its author. If we can believe that every
structural arrangement in plants, which depends upon certain external
conditions, has been produced in a phyletic sense by these latter, it
becomes very easy to explain the transformation of species; but in
accepting such an explanation we are building without any foundation,
for the proof that acquired characters can be transmitted has yet to be
given.

As a further disproof of my views Detmer quotes the so-called phenomena
of correlation in plants, and he believes that these instances help us
to conceive how the acquired changes of the body (_soma_) of the plant
may also influence the sexual cells. If the apical shoot of a young
spruce fir be cut off, one of the lateral shoots of the whorl next
below the section rises and becomes an apical shoot: it not only
assumes the orthotropic growth of such a shoot, but also its mode of
branching. The phenomenon itself is well known, and I have often
observed it myself in my garden without making any botanical
experiments; for this experiment is not uncommonly made by Nature
herself, when the apical shoot is destroyed by insects (for example the
gall-making _Chermes_). The change of the lateral into an apical shoot
occurs here in consequence of the loss of the true apical shoot, and is
therefore really dependent upon it. The only difficulty is to
understand how these and many other kindred phenomena can be considered
to prove the transmission of acquired characters. That correlation
exists between the parts of an organism, that correlated changes are
not only common but nearly always accompany some primary change, has
been perfectly well known since Darwin’s time, and I am not aware that
it has been disputed by any one. I further believe that hardly any one
would maintain that it is impossible for the reproductive organs to be
influenced by correlation. But this is very far from the admission that
such changes would occur in the germ-cells as would be necessary for
the transmission of acquired characters. For such transmission to occur
it would be necessary for the germ-plasm (the bearer of hereditary
tendencies) to undergo a transformation corresponding to that produced
by the external influences;—such a transformation as would cause the
future organism to spontaneously develope changes similar to those
which its parent had acquired. But since the germ-plasm is not an
organism in the sense of being a microscopic facsimile which only has
to increase in size in order to become a mature organism, it is obvious
that the developmental tendencies must exist in the specific molecular
structure, and perhaps also in the chemical constitution of the
germ-plasm itself. It therefore follows that the changes in the
germ-plasm which would be required for the transmission of an acquired
character must be of an entirely different nature from the change
itself acquired by the body of the parent plant: and yet it is supposed
that the former is produced by the latter as a result of correlation. I
will illustrate this by an example. Let us suppose that the influence
of climate had caused a plant to change the form of its leaves from an
ovate into a lobate shape: now such a change could not be transferred
to the germ-plasm in the pollen and the ovules, as anything similar to
leaves or the form of leaves; for such specialized morphological
features have no existence in the germ-plasm. The only thing which
could happen would be changes in its molecular structure which bear no
resemblance to those changes which are implied by the direct alteration
of the form of the leaf in the parent plant. Any one who clearly
appreciates this difficulty will hesitate in admitting the possibility
of the transmission of acquired characters, because it is possible that
the sexual cells may be affected by correlated influences. If the
change in the form of a leaf exercises any influence at all upon the
germ-plasm, why should it produce a corresponding (in the
above-mentioned sense) change in its molecular structure? Why should it
not produce some other out of the immense number of possible changes?
There must be as many possible changes in the structure of germ-plasm
as there are possible variations in each part of a plant that arises
from it. Why then should the corresponding change always occur,—a
change which had never previously existed in the whole phyletic
development of the organic world; for the plant with the latest
modification can have never existed before? The occurrence of a
particular change out of the countless possible changes would be about
as likely as if one out of a hundred thousand pins thrown out of a
window were to balance on its point when it reached the ground. The
assumption scarcely deserves to be called a scientific hypothesis, and
yet it must be made by all who accept the transmission of acquired
characters,—that is unless they adopt the hypothesis of pangenesis,
which is quite as improbable, and which even Darwin did not look upon
as a real, but only as a formal explanation.

Detmer is also greatly mistaken when he says that I refuse to admit the
transmission of acquired characters, because I am prejudiced in favour
of my doctrine of the continuity of the germ-plasm. This doctrine is
either right or wrong, and there is no middle course: to this extent I
quite admit that I am prejudiced. But the question as to whether
acquired characters can be impressed upon the germ and thus transmitted
would not be by any means settled in this way; for even if we admit
that the germ-plasm is not continuous from one generation to another,
but that it must be produced afresh in each individual, this would by
no means necessarily imply that it would potentially receive and retain
every change produced in every part of the individual, and at any time
in its life. It seems to me that the problem of the transmission or
non-transmission of acquired characters remains, whether the theory of
the continuity of the germ-plasm be accepted or rejected.

I will now proceed to examine the last group of phenomena which Detmer
brings forward in favour of the transmission of acquired characters. He
charges me with not having taken into account, in discussing the
problem of heredity, the very important facts which are known about the
strange phenomena of ‘after-effect’ in plants. Among these
‘after-effects’ are the following.

If vigorous plants of the sun-flower, grown in the open air, be cut off
close to the ground and transferred to complete darkness, the
examination of a tube fixed to the cut surface of the stem will show
that the escape of sap does not take place uniformly, but undergoes
periodical fluctuation, being strongest in the afternoon and weakest in
the early morning. Now the cause of this daily periodicity in the flow
of sap depends upon the periodical changes due to the light to which
the plant was exposed when it was growing under normal conditions. When
plants which have been grown in darkness from the first are similarly
treated, the flow of sap does not exhibit any such periodicity.

Another instance is as follows:—it is well known that darkness
accelerates, while light retards the growth of plants, and therefore
plants usually grow more strongly by night than by day. If now plants
are transferred from the open air into constant darkness, the
periodicity in their growth does not immediately disappear, and often
persists for a long time as a phenomenon of after-effect.

The opening and closing of the leaves of _Mimosa pudica_ also takes
place periodically under natural conditions, the leaves closing at dusk
as a result of changes in the stimulus provided by the light. In this
case also, when the plants are transferred to constant darkness, the
periodicity in the movements of the leaves continues for several days.

All this is certainly very interesting, and it proves that periodical
stimuli produce periodical processes in the plant, which are not
immediately arrested when the stimulus is withdrawn, and only become
uniform gradually and after the lapse of a considerable time. But I
certainly claim the right to ask what connexion there is between these
facts and the transmission of acquired characters. All these
peculiarities produced by external influences remain restricted to the
individual in which they arose; most of them disappear comparatively
soon, and long before the death of the individual. No example of the
transmission of such a peculiarity is known. Although successive
generations of sunflowers have been exposed for thousands of years to
the daily alternation of light and darkness, the periodicity in the
flow of sap has not become hereditary, and does not take place at all
in plants which have always been kept in darkness. Detmer specially
tells us that we can even reverse the periods of opening and closing
the leaves in _Mimosa pudica_ by keeping them in darkness during the
day, but exposed to light at night; an experiment which was performed
by Pfeffer. Here again we see the proof that influences which have
acted upon countless generations have left no impression whatever upon
the germ-plasm.

Detmer himself admits this when he says that the after-effects are only
witnessed during the life of the individual, but he nevertheless adds
that he has been for many years convinced that the phenomena of
heredity and after-effect differ in degree and not in kind. He even
goes so far as to assert that, in spite of the obvious non-transmission
of after-effect, the similarity between the natures of these two
classes of phenomena cannot escape the intelligent observer.

It seems to me that this question does not demand the attention of the
observer (for the observations have already been made) so much as that
of the thinker. It is not a correct train of reasoning to conclude that
after-effect and heredity are identical in nature, from the fact that
certain periodical influences, acting upon a single individual, set up
periodical physiological processes which continue for a time after the
influences have ceased to act. We might almost as well argue that the
oscillations of a pendulum, which continue as after-effects when the
pendulum has been set going, are of an identical nature with the
process of heredity. All these phenomena have indeed this much in
common:—a cause which acted at some time in the past, but which is no
longer visible at the time when the phenomenon appears. But the
likeness ends here, and the supposed identity in nature merely depends
upon wild speculation. One difference is very obvious, for the
phenomena of after-effect gradually cease after the withdrawal of the
stimulus, just like the oscillations of the pendulum, while the
phenomena of heredity continue without any interruption. As far as
heredity is concerned the physiological processes of after-effect are
not distinguishable from any of the other well-known acquired
characters which are recognizable as morphological changes.
After-effects are not transmitted, and compared with this fact but
little importance can be attached to the use of vague analogies by
Detmer, who would wish to conclude that heredity is only the
after-effect of processes which had been set going in the parent
organism.

At the end of his paper Detmer applies the ideas which he has gained
from the consideration of after-effect to certain phenomena in the
normal life of plants. He suggests that the periodical change of leaf
in trees and shrubs may have been produced by the direct effect of
climate. If branches bearing winter buds are cut off in the autumn and
are placed in a hot-house, with their cut ends in water, the buds do
not at once develope, and months may often elapse before they begin to
break. He argues that this experiment proves that the annual
periodicity of the plant no longer depends directly upon external
influences; these latter produced the periodicity at some earlier time,
but it has been gradually fixed in the organism by after-effect and
heredity(!), so that its disappearance does not now take place when the
stimulus is withdrawn, and changes would only happen very gradually
under the influence of changed climatic conditions. He considers that
this is proved from the fact that our cherry has become an evergreen in
Ceylon.

Such are Detmer’s opinions, and every one will agree with him in
believing that the periodical change of leaf in temperate climates has
been produced in relation to the recurring alternation of summer and
winter. This is certainly the case, and it cannot be doubted that the
character has become fixed by heredity. Where, however, is the proof
that this hereditary character has been produced by the direct
influence of climate? What right have we to look upon the hereditary
appearance of the character as an after-effect of the direct influence
exerted by changes of temperature upon previous generations? Such an
opinion derives but little support from the previously described
experiments upon after-effect, which showed that these phenomena were
never hereditary.

It appears to me that there are certain points in this change of leaf
and its accompanying phenomena, which distinctly indicate that natural
selection has been at work. Can Detmer imagine that the brown scales
which form the characteristic protective covering of winter buds have
been produced by the direct action of the cold? If, however, the
peculiar structure of these buds is to be referred to the specific
constitution of the individual rather than to the direct effects of
climate, would it be so very improbable for their physiological
peculiarity of lying dormant for several months to have been developed
simultaneously with the structure, by the operation of natural
selection? And if this explanation be correct, we can at once see why
the character has become hereditary, for natural selection works upon
variations of the germ-plasm, and these are transferred from one
generation to another with the germ-plasm itself.

But Detmer attempts to establish the converse conclusion, and he argues
that the hereditary change of leaf has been abandoned under the
long-continued effect of changed climatic conditions; but this opinion
is based upon the single instance of the alteration in the habit of the
European cherry in Ceylon. If it were proved that our cherry, grown
from seed in Ceylon and propagated by seed for several generations,
became evergreen gradually and not suddenly in the first generation:
if, under such circumstances, it came to retain its leaves in the
autumn and ceased to produce the dormant winter buds:—then indeed the
transmission of acquired characters could hardly be doubted. I am not a
botanist, but I believe I am right in supposing that the wild cherry
reproduces itself by seeds, while the edible domesticated cherry is
propagated by grafting. Grafts are, however, parts of the _soma_ of a
previously existent tree, and we are not therefore concerned, in this
method of propagation, with a succession of generations, but with the
successive distribution of one and the same individual over many wild
stocks. But no one will doubt that one and the same individual can be
gradually changed during the course of its life, by the direct action
of external influences. The really doubtful point is whether such
changes can be transmitted by means of the germ-cells. If, as I
presume, the English in Ceylon do not care to eat wild cherries but
prefer the cultivated kinds, it follows that the branches which bear
fruit in that island have not been developed from germ-cells, at any
time since their introduction, and there is nothing to prevent them
from gradually changing their anatomical and physiological characters
in consequence of the direct influence of climate.

Hence the instance which Detmer looks upon as plainly conclusive, can
hardly be accepted in support of such a far-reaching assumption as the
transmission of acquired characters.

It is therefore clear that none of the facts brought forward by Detmer
really afford the proofs which he believes that they offer. But another
botanist, Professor Hoffman of Marburg, well known for his
long-continued experiments on variation, has recently called attention
to certain other botanical facts in support of the transmission of
acquired characters. These facts are indeed conclusive, if we accept
the author’s use of the term ‘acquired,’ but it will be found that they
lead to hardly any modification in the state of existing opinion upon
the subject.

In a short note, dated Jan. 1, 1888, the author communicated to this
journal (‘Biologisches Centralblatt’) the statement that changes in the
structure of flowers caused by poor nutrition can be proved to be
hereditary to a greater or less extent[288].

A more elaborate account of the experiments will be found in several
numbers of the ‘Botanische Zeitung,’ and the author expresses his final
results in the following words (see Bot. Zeit. 1887, p. 773):—‘These
experiments prove with certainty (1) that insufficient nutrition may
cause considerable morphological changes (viz. qualitative variations)
which are in the first place acquired by the sexual apparatus of the
flower, (2) that the “transient” (Weismann) characters acquired by the
individual can be transmitted[289].’

The data upon which Hoffman bases these opinions are certain
experiments conducted upon various plants, in order to determine the
conditions of life under which abnormal flowers or any other variations
occur most frequently: to decide, in short, how far variations are
caused by the change of conditions.

It is obvious that the attention of the author was not at first
directed to the question of the transmission of acquired characters.
His experiments are of a much older date than the present condition and
significance of the question before us. Hoffmann has, in fact,
re-examined his former results from the new point of view, and this
explains why his proofs are not always sufficiently convincing when
applied to the present issue. But this is of no great importance,
inasmuch as there is no necessity for me to question the correctness of
his assumptions.

The essential details of the experiments to which he directs attention
are as follows.

Different plants with normal flowers were subjected to greatly changed
conditions of life for a series of generations. They were, for example,
crowded together in small pots. Under these circumstances the plants
were of course poorly nourished, and in the course of generations,
several species produced a variable proportion of abnormal—viz.
double-flowers. This, however, was not always the case, for such
flowers did not appear in _Matthiola annua_ and _Helianthemum
polifolium_. In other species, such as _Nigella damascena_, _Papaver
alpinum_ and _Tagetes patula_, they appeared and often increased in
numbers in the course of generations, although this was not a constant
result. For instance, four successive generations of _Nigella
damascena_, when closely sown, produced the following results:—

              1883. No double flowers.
              1884. No double flowers.
              1885. 23 typical flowers: 6 double flowers.
              1886. 10 typical flowers: 1 double flower.

But it was not always the case that the double flowers continued to
appear after they had been once produced. In _Papaver alpinum_, which
Hoffman has cultivated in successive generations since 1862, other
changes in addition to the doubling of the flowers first appeared in
1882, viz. a slight variability in the form of the leaf, and a greater
variability in the colours of the flowers. The production of double
flowers appeared to be favoured by poor nutrition caused by crowding
the plants. The results as regards the number of double flowers
produced in this species by close sowing, from 1882-1886, have been as
follows:—

       Experiment XI.   1881. 40  per cent. of double flowers.
                        1882. 4   per cent. of double flowers.
                        1883. 5·3 per cent. of double flowers.

       Experiment XVII. 1884. 13· per cent. of double flowers.
                        1885. 0·0 per cent. of double flowers.
                        1886. 0·0 per cent. of double flowers.

Although in these and some other series of generations the double
flowers again disappeared in the later generations, yet there can be
hardly any doubt that their first appearance was due to the abnormal
conditions of nutrition. This conclusion is also unaffected by the fact
that double flowers appeared in nearly the same proportions in
consequence of cultivation in ordinary garden soil. The plants which
were crowded in pots produced 2879 normal flowers, and 256 (=8·8 per
cent.) abnormal and mostly double ones, while 867 normal and 62 (=7·0
per cent.) abnormal ones were produced on garden beds. Hoffman will not
indeed admit that such a comparison can be fairly made, for the plants
in the garden beds were raised from seed which was in part taken from
the double flowers, and was therefore, he believed, under a strong
hereditary influence. But this latter assumption is not supported by
the results of his own experiments.

Thus experiment XVIII., conducted upon _Papaver alpinum_, is described
in these words,—‘Seeds yielded by double flowers from experiment XI.
(1883), were sown in pots, and the resulting plants produced from
1884-1886, fifty-three single flowers and no double ones.’

In the converse experiment XIX. ‘The seeds of single flowers from
different stocks were sown in pots, and the resulting plants produced
in 1885 and 1886 forty-three flowers, of which all were typical except
one;’ while plants produced in the garden by seed from the same
sources, yielded 166 single and five double flowers. Hoffman also
describes other experiments in which the seeds from double flowers
produced plants which also yielded many double flowers. Thus, for
example, in experiment XXI. seeds yielded by the double flowers of
_Papaver alpinum_ were sown in the garden and produced numerous plants,
which in 1885 and 1886 bore 284 single and twenty-one double flowers,
that is 7 per cent. of the latter.

It will therefore be seen that the transmission of the abnormality is
by no means proved beyond the possibility of doubt, for who can decide
between the effects due to heredity and changed conditions in the last
experiment? I have no doubt however that the results are at any rate in
part due to the operation of heredity, for I do not see how the
phenomena can be otherwise understood. Nevertheless I cannot admit the
transmission of acquired characters on this evidence, for the changes
which have appeared are not ‘acquired’ in the sense in which I use the
term and in the sense required by the general theory of evolution. It
is true that they may be described by the use of this word: inasmuch as
they are characters which the plant has come to possess; we are not
however engaged in a mere dispute about terms, but in the discussion of
a weighty scientific question. Our object is to decide whether changes
in the _soma_ (the body, as opposed to the germ-cells) which have been
produced by the direct action of external influences, including use and
disuse, can be transmitted; whether they can influence the germ-cells
in such a manner that the latter will cause the spontaneous appearance
of corresponding changes in the next generation. This is the question
which demands an answer; and, as has been shown above, such an answer
would decide whether the Lamarckian principle of transformation must be
retained or abandoned.

I have never doubted about the transmission of changes which depend
upon an alteration in the germ-plasm of the reproductive cells, for I
have always asserted that these changes, and these alone, must be
transmitted. If any one makes the contrary assertion, he merely proves
that he does not understand what I have said upon the subject. In what
other way could the transformation of species be produced, if changes
in the germ-plasm cannot be transmitted? And how could the germ-plasm
be changed except by the operation of external influences, using the
words in their widest sense; unless indeed we assume with Nägeli, that
changes occur from internal causes, and imagine that the phyletic
development of the organic world was planned in the molecular structure
of the first and simplest organism, so that all forms of life were
compelled to arise from it, in the course of time, and would have
arisen under any conditions of life. This is the outcome of Nägeli’s
view, against which I have contended for years.

If we now use the term ‘acquired characters’ for changes in the soma
which, like spontaneous abnormalities, depend upon previous changes in
the germ-plasm—it is of course easy to prove that acquired characters
are transmitted; but this is hardly the way to advance science, for
nothing but confusion would be produced by such a use of terms[290]. I
am not aware that any one has ever doubted that spontaneous characters,
such as extra fingers or toes, patches of grey hair, moles, etc., can
be transmitted. It is true that such characters are sometimes called
‘acquired’ in pathological works, but His has rightly insisted that
such an obviously inaccurate use of the term ought to be avoided, in
order to prevent misunderstanding. If every new character is said to be
‘acquired’ the term at once loses its scientific value, which lies in
the restricted use. If generally used, it would mean no more than the
word ‘new’; but new characters may arise in various ways,—by artificial
or natural selection, by the spontaneous variations of the germ, or by
the direct effect of external influences upon the body, including the
use and disuse of parts. If we assume that these latter characters are
transmitted, the further ‘assumption of complicated relations between
the organs and the essential substance of the germ becomes necessary’
(His), while the transmission of the other kinds of characters do not
involve any theoretical difficulties. There is therefore obviously a
wide difference between these two groups of characters as far as
heredity is concerned, quite apart from the question as to whether
acquired characters are really transmitted. It is at all events
necessary to have distinct terms which cannot be misunderstood.
His[291] has proposed to call those characters which are due to
selection ‘changes produced by breeding’ (‘erzüchtete Abänderungen’),
those which appear spontaneously—‘spontaneous changes’ (‘eingesprengte
Abänderungen’), and these two groups of characters would then be
opposed to those which he calls ‘acquired changes’ (‘erworbene
Abänderungen’), of course using the term in the restricted sense.
Science has always claimed the right of taking certain expressions and
applying them in a special sense, and I see no reason why it should not
exercise this right in the case of the term ‘acquired.’ It appears
moreover that this word has not always been used in this vague sense by
pathological anatomists, such as Virchow and Orth; for Weigert and
Ernst Ziegler have employed it in precisely the same sense as that in
which it has been used by Darwin, du Bois-Reymond, Pflüger, His and
many others, including myself.

It is certainly necessary to have two terms which distinguish sharply
between the two chief groups of characters—the primary characters which
first appear in the body itself, and the secondary ones which owe their
appearance to variations in the germ, however such variations may have
arisen. We have hitherto been accustomed to call the former ‘acquired
characters,’ but we might also call them ‘_somatogenic_,’ because they
follow from the reaction of the _soma_ under external influences; while
all other characters might be contrasted as ‘_blastogenic_,’ because
they include all those characters in the body which have arisen from
changes in the germ. In this way we might perhaps prevent the
possibility of misunderstanding. We maintain that the ‘_somatogenic_’
characters cannot be transmitted, or rather, that those who assert that
they can be transmitted, must furnish the requisite proofs. The
_somatogenic_ characters not only include the effects of mutilation,
but the changes which follow from increased or diminished performance
of function, and those which are directly due to nutrition and any of
the other external influences which act upon the body. Among the
_blastogenic_ characters, we include not only all the changes produced
by natural selection operating upon variations in the germ, but all
other characters which result from this latter cause.

If we now wish to place Hoffmann’s results in their right position, we
must regard all of them as ‘_blastogenic_’ characters, for no one of
them can be considered as belonging to the group which has been
hitherto spoken of as ‘acquired,’ in the literature of evolution: they
are not due to _somatogenic_ but to _blastogenic_ changes. The body of
the plant—the _soma_—has not been directly affected by external
influences, in Hoffman’s experiments, but changes have been wrought in
the germ-plasm of the germ-cells and, only after this, in the _soma_ of
succeeding generations.

There is no difficulty in finding facts in support of this statement,
among Hoffmann’s experiments. The proof chiefly lies in the fact that
in no one of his numerous experiments did any change appear in the
first generation. The seeds of different species of wild plants, with
normal flowers, were cultivated in the garden and in pots (thickly sown
in the latter case), but no one of the plants produced by these wild
seeds possessed a single double flower. It was only after a greater or
less number of generations had elapsed that a variable proportion of
double flowers appeared, sometimes accompanied by changes in the leaves
and in the colours of the flowers. This fact admits of only one
interpretation;—the changed conditions at first produced slight and
ineffectual changes in the idioplasm of the individual, which was
transmitted to the following generation: in this again the same causes
operated and increased the changes in the idioplasm which was again
handed down. Thus the idioplasm was changed more and more, in the
course of generations, until at last the change became great enough to
produce a visible character in the _soma_ developed from it, such as,
for example, the appearance of a double flower. Now the idioplasm of
the first ontogenetic stage (viz. germ-plasm) alone passes from one
generation to another, and hence it is clear that the germ-plasm itself
must have been gradually changed by the conditions of life until the
alteration became sufficient to produce changes in the _soma_, which
appeared as visible characters in either the flower or leaf[292].

In addition to the above-mentioned cases Hoffmann also quotes some
facts of a somewhat different kind. He succeeded in inducing
considerable changes in the structure of the root of the wild carrot
(_Daucus carota_) by means of the changes in nutrition implied by
garden cultivation. These changes also proved to be hereditary.

Unfortunately, I have not the literature of the subject at hand, and
hence I am unable to read the accounts of these older experiments _in
extenso_; but it is sufficiently obvious that in this case we are also
concerned with a change which did not become visible until after some
generations had elapsed, and which was therefore a change in the
germ-plasm.

Many instances of a precisely similar kind have been long known, and
one of them is to be found in the history of the garden pansy, which
Hoffmann has succeeded in producing from the wild form, _Viola
tricolor_, in the course of eighteen years. Darwin some time ago
pointed out in his work upon ‘The Variation of Animals and Plants under
Domestication,’ that, in the case of the pansy and all other ‘improved’
garden flowers, the wild form remained unchanged for many generations
after its transference to the garden, apparently uninfluenced by the
new conditions of life. At length single varieties began to appear, and
these were further developed by artificial selection and appropriate
crossing, into well-marked races distinguished by peculiar colours,
forms, etc.

In these cases also, changes in the germ-plasm are the first results of
the new conditions, and there is no evidence for the occurrence of
acquired characters, using the term in its restricted sense.

I now come to the last botanical fact brought forward by Hoffmann in
support of the transmission of acquired characters. He states that
specimens of _Solidago virgaurea_ brought from the Alps of the Valais,
commenced flowering in the botanical garden at Giessen, at a time which
differed by several weeks from that at which specimens from the
surrounding country, planted beside them, began to flower. In other
words, the time of flowering must have been fixed by heredity in the
alpine _Solidago_, for the external conditions would have favoured a
time which was simultaneous with that of the Giessen plants.

What conclusions can be drawn from these facts? Hoffmann of course sees
in them the proof of the transmission of acquired characters, but this
presupposes that the time of flowering was originally an acquired
character. Hoffmann indeed appears to entertain this opinion when he
somewhat vaguely states that the time at which flowering begins has
been acquired by accommodation—that is by the influence of
climate—during a long series of generations, and has become hereditary.
But what does Hoffmann mean by ‘accommodation’? He presumably means
that which, since the appearance of Darwin’s writings, has been
generally called adaptation:—that is a purposeful arrangement, suited
to certain conditions. The majority of biologists have followed Darwin
in believing that such adaptations have been produced by processes of
natural selection. Hoffmann seems to imagine that they have arisen in
some other way: perhaps he believes, with Nägeli, that they have been
directly produced by external influences.

The fixation of the time at which flowering begins, is an adaptation
which formerly could have been very well explained as the direct result
of external conditions. The question we have to decide is whether such
an explanation is the true one. We might imagine that the plant would
be forced into quicker development by an earlier appearance of the warm
season. Hence when transferred into a warmer climate the plant would at
first flower rather earlier, the habit would then be transmitted, and
would increase in successive generations from the continued influence
of climate, until it advanced as far as the organization of the plant
permitted. But in this explanation, as in so many others of the same
kind, it has unfortunately been forgotten that the transmission of
acquired characters which is presupposed in the explanation is a
totally unproved hypothesis. It is sufficiently obvious that by
interpreting a phenomenon in a manner which presupposes the
transmission of acquired characters, we cannot furnish a proof of the
existence of such transmission.

It always seemed to me that the fixation of the commencement of
flowering, together with similar physiological phenomena in the animal
kingdom (for example, the hatching of insects from winter eggs), could
be explained very satisfactorily by the operation of natural selection:
and even now this explanation appears to me to be the simplest and most
natural. In Freiburg, where the vine is largely grown, the harvest is
often injured by frosts in spring, which kill the young shoots, buds
and flowers. Accordingly, different kinds of vine, which do not push
their buds so early, have now been planted. Any one, who has seen all
the shoots of the former destroyed by the frosts at the end of April,
while the latter, not having opened their buds, were spared, would not
doubt that the former must have been long ago exterminated, if they had
been compelled to struggle for existence with the others, under natural
conditions. Now the time of flowering fluctuates slightly in the
individuals of every species of plant, and can therefore be modified by
natural selection. It is therefore difficult to see why the time at
which each plant flowers should not have been fixed in the most
favourable manner for each habitat, by natural selection alone.

Hoffmann is obviously unaware of the fundamental distinction between
the characters primarily acquired by the _soma_, and the secondary
characters which follow from changes in the germ-plasm.

If the author had appreciated this distinction he would not have
attempted to strengthen his opinions by following up the botanical
facts which exclusively belong to the second class of characters, with
the enumeration of certain instances selected from the animal kingdom
(viz., the supposed transmission of mutilations), all of which belong
to the first class. I will not discuss these latter instances, for most
of them are old friends, and they are all far too uncertain and
inaccurate to have any claim on scientific consideration.

I believe that I have shown that no botanical facts have been hitherto
brought forward which prove the transmission of acquired characters (in
the restricted sense), and that there are not even any facts which
render such transmission probable.

                                                             A. W.

    Naples, Zoological Station,

        _Jan. 11, 1888_.

------------------------------------------------------------------------

                        Footnotes for Essay VII.

Footnote 275:

  See the second Essay.

Footnote 276:

  Consult ‘Ueber die Vererbung,’ Jena, 1883; ‘Die Kontinuität des
  Keimplasmas,’ Jena, 1885; ‘Ueber die Zahl der Richtungskörper und
  über ihre Bedeutung für die Vererbung,’ Jena, 1887. These papers are
  translated as the second, fourth and sixth Essays in the present
  volume.

Footnote 277:

  See the second Essay.

Footnote 278:

  [See R. Meldola in Ann. and Mag. Nat. Hist., 1878, vol. i. pp.
  158-161. The author discusses many cases among insects in which
  instinct is related to protective structure or colouring: he also
  considers that instinct is to be explained by the principle of
  natural selection which accounts for the other protective
  features.—E. B. P.]

Footnote 279:

  [See ‘Nature,’ vol. 36, pp. 491-507.—E. B. P.]

Footnote 280:

  [See ‘The Factors of organic Evolution’ in ‘The Nineteenth Century’
  for April and May 1886.—E. B. P.]

Footnote 281:

  See ‘Biol. Centralbl.’ Bd. VII. No. 23.

Footnote 282:

  See the next Essay (VIII).

Footnote 283:

  Detmer, ‘Zum Problem der Vererbung,’ Pflüger’s Archiv f. Physiologie,
  Bd. 41, (1887), p. 203.

Footnote 284:

  [Dr. Weismann is here alluding to experiments upon the larvae of
  _Rumia Crataegata_. A short account of the results will be found in
  the Report of the British Association at Manchester (1887), and in
  ‘Nature,’ vol. 36, p. 594. I have now obtained similar results with
  many other species (see Trans. Ent. Soc., Lond. 1888, p. 553); but
  many of the results are as yet unpublished.—E. B. P.]

Footnote 285:

  [See the editorial notes by Raphael Meldola, in his translation of
  Weismann’s ‘Studies in the Theory of Descent’ (the Essay on ‘The
  Origin of the Markings of Caterpillars,’ pp. 241 and 306): also E. B.
  Poulton, in ‘Proc. Roy. Soc.,’ vol. xxxviii. pp. 296-314; and in
  ‘Proc. Roy. Soc.,’ vol. xl. p. 135.—E. B. P.]

Footnote 286:

  [Professor Meldola first called attention to the scattered instances
  of the kind here alluded to by Professor Weismann, in 1873: see
  ‘Proc. Zool. Soc.,’ 1873, p. 153. The author explains the relation of
  this ‘variable protective colouring’ to other protective appearances,
  and he is strongly of the opinion that the former as well as the
  latter is to be explained by the action of the ‘survival of the
  fittest.’

  The validity of Dr. Weismann’s interpretation of these effects as due
  to adaptation, through the operation of natural selection, is
  conclusively proved by the following facts. The light reflected from
  green leaves becomes the stimulus for _the production of dark brown
  pigment_ in those cases in which the leaves constitute the
  surroundings for many months. Under these circumstances the leaves of
  course become brown at a relatively early date, and protection is
  thus afforded for the remainder of the period, although the dark
  pigment is produced before the change in the colour of the leaf.
  Instances of this kind are seen in the colours of cocoons spun among
  leaves by certain lepidopterous larvae (see ‘Proc. Ent. Soc. Lond.,’
  1887, pp. l, li, and 1888, p. xxviii), the cocoons of the same
  species being of a creamy white colour when spun upon white paper.

  Conversely, the light reflected from the same surfaces serves as the
  stimulus for _withholding pigment_ in the cases alluded to by Dr.
  Weismann (larvae of _R. Crataegata_, &c.), in all of which the
  organism only remains in contact with the leaves while they are
  green, viz. at a time when the dark colour would be disadvantageous.

  Hence precisely opposite effects are produced by the operation of the
  same force; the nature of the effect which actually follows in any
  case being solely determined by the advantage afforded to the
  organism.—E. B. P.]

Footnote 287:

  Compare Sachs, ‘Lectures on the Physiology of Plants,’ translated by
  H. Marshall Ward, p. 710.

Footnote 288:

  Compare Biol. Centralbl. Bd. VII. No. 21.

Footnote 289:

  I have used the expression ‘transient’ (‘passant’) in the same sense
  as ‘acquired,’ in order to enforce the conclusion that they are
  merely temporary, and disappear with the individual in which they
  arise. Since the characters of which Hoffmann speaks are hereditary,
  the term cannot be rightly applied to them, and I shall prove later
  on that they cannot be regarded as acquired characters in the sense
  required by the theory of descent.

Footnote 290:

   Compare a paper by J. Orth, ‘Ueber die Entstehung und Vererbung
  individueller Eigenschaften,’ Leipzig, 1887. This author considers my
  theory of the non-transmission of acquired characters to be
  incorrect, because he will insist upon using the term ‘acquired’ for
  those characters which are due to spontaneous changes in the germ;
  although he considers that they are only indirectly acquired. He also
  reproaches me with not having discriminated with sufficient clearness
  between the two modes in which new characters are acquired by the
  body, and with having altogether failed to take into account the
  class of characters which are due to variations in the germ. On the
  very same page he quotes the following sentence from my
  writings:—‘Every change of the germ-plasm itself, however it may have
  arisen, must be transmitted to the following generation by the
  continuity of the germ-plasm; and hence also any changes in the
  _soma_ which arise from the germ-plasm must be transmitted to the
  following generation.’ Not only does the transmission of Orth’s
  ‘indirectly acquired characters’ necessarily follow from this
  sentence, but it is even distinctly asserted by it. I cannot
  understand how any one who is aware of what happened at the meeting
  of the Association of German naturalists at Strassburg in 1885, can
  charge me with the confusion of ideas which has prevailed since
  Virchow took part in the discussion of this question.

Footnote 291:

  His, ‘Unsere Körperform,’ Leipzig, 1874, p. 58.

Footnote 292:

  Compare on this point Nägeli in his ‘Theorie der Abstammungslehre.’
  This writer also concludes from similar facts that external
  influences have wrought in the idioplasm, changes which were at first
  ineffectual, and which only increased during the course of
  generations up to a point at which they could produce visible changes
  in the plant. He does not, however, draw the further conclusion that
  these changes only influence the germ-plasm, for he was not aware of
  the distinction between germ-plasm and somatoplasm.

------------------------------------------------------------------------






                                 VIII.


                      THE SUPPOSED TRANSMISSION OF
                              MUTILATIONS.


                                 1888.



        A lecture delivered at the Meeting of the Association of
             German Naturalists at Cologne, September 1888.







------------------------------------------------------------------------




                                 VIII.

                      THE SUPPOSED TRANSMISSION OF
                              MUTILATIONS.


We know well the manner in which Lamarck imagined that the gradual
transformation of species occurred, when he first made the attempt to
penetrate into the mechanism of the process of evolution, and to
ascertain the causes by which it is produced. In his opinion, a change
in the structure of any part of an organism was chiefly brought about
when the species in question met with new conditions of life and was
thus forced to assume new habits. Such habits caused an increased or
diminished activity, and therefore a stronger or weaker development, of
certain parts, and the modified parts were then transmitted to the
offspring. Inasmuch as the offspring continued to live under the same
changed conditions, and kept up the altered manner of using the part in
question, the inherited changes would be increased in the same
direction during the course of their life, and would be further
increased in each successive generation, until the greatest possible
change had been effected.

In this way Lamarck was able to give an apparently satisfactory
explanation of at any rate those changes which consist in the mere
enlargement or diminution of a part; such, for instance, as the great
length of neck in the swan and other swimming birds, which he believed
to have been produced by the habit of stretching after food at the
bottom of the water; or the webbed feet of the same animals, supposed
to be produced by the habit of striking the water with outspread toes,
etc. In this way he was also able to explain the disappearance of a
part after it had ceased to be of use; as, for instance, the
degeneration of the eyes of animals inhabiting caves or the sunless
depths of lakes or the sea.

But it is obvious that such an explanation tacitly assumes that changes
produced by use or disuse can be transmitted to the offspring; _it
assumes the transmission of acquired characters_.

Lamarck made this assumption as a matter of course, and when half a
century later Charles Darwin, his more fortunate successor, refounded
the theory of organic evolution, he also believed that we could not
entirely dispense with the Lamarckian principle of explanation,
although he added the new and extremely far-reaching principle of
natural selection. But he certainly attempted to decide whether the
Lamarckian principle of the effects of use and disuse is truly
efficient, by asking himself the question whether such changes, as for
example those produced by exercise during an individual life, can be
transmitted to the offspring. Many observations appeared to him, if not
to prove the transmission directly, yet to render it extremely
probable; and he thus came to the conclusion that there is no
sufficient reason for denying the transmission of acquired changes.
Hence, in Darwin’s works, use and disuse still play important parts as
direct factors of transformation, in addition to natural selection.

Darwin was not only an original genius, but also an extraordinarily
unbiassed and careful investigator. Whatever he expressed as his
opinion had been carefully tested and considered. This impression is
gained by every one who has studied Darwin’s writings, and perhaps it
in part explains the fact that doubts as to the correctness of the
Lamarckian principle adopted by him have only arisen during the last
few years. These doubts have, however, culminated in the decided denial
of the assumption that changes acquired by the body can be transmitted.
I for one frankly admit that I was in this respect under the influence
of Darwin for a long time, and that only by approaching the subject
from an entirely different direction was I led to doubt the
transmission of acquired characters. In the course of further
investigations I gradually gained a more decided conviction that such
transmission has no existence in fact.

Doubts on this point have been expressed not only by me but also by
others, such as du Bois-Reymond and Pflüger. Indeed, concerning a
certain class of acquired characters, viz. mutilations, the great
German philosopher, Kant, has distinctly denied that transmission can
take place[293]; and in more recent times Wilhelm His has expressed the
same opinion[294].

But if the transmission of acquired characters is truly impossible our
theory of evolution must undergo material changes. We must completely
abandon the Lamarckian principle, while the principle of Darwin and
Wallace, viz. natural selection, will gain an immensely increased
importance.

When I first expressed this opinion in my essay ‘On Heredity[295],’ I
was well aware of the consequences of such an idea. I knew well that
apparently insurmountable obstacles would be raised against any
explanation of evolution, from which the principle of the direct
transformation of the species by external influences had been excluded.
I therefore endeavoured to show that these difficulties are not in
reality insurmountable, and that it is quite possible to explain
certain phenomena, such as the degeneration of useless parts, without
the aid of the Lamarckian principle. Furthermore it can be shown that a
not inconsiderable number of instincts, viz. all those which are
exercised only once in a lifetime, cannot possibly have arisen by
transmitted practice. This fact renders it unnecessary to make use of
the Lamarckian principle for the explanation of other kinds of
instinct. I do not mean to deny the existence of phenomena for which
such an explanation has not yet been found, or at least has not been
brought forward; but on the other hand it appears to me that it has
never been proved that we cannot dispense with the Lamarckian principle
in the explanation of these phenomena. At any rate, I do not know of
any facts which could induce us to abandon from the first any hope of
finding an explanation without the aid of this hypothesis.

If we are able to prove that we may dispense with the assumption of the
transmission of acquired characters in explaining such phenomena, of
course it by no means follows that we _must_ dispense with it; or, in
other words, it does not follow that the transmission of acquired
changes cannot take place. It would be as unsafe to make this assertion
as to state of a ship seen at a great distance, that it is only moving
by sails and not by steam simply because the movement appears to be
explicable by sails alone. We ought first to attempt to show that the
ship does not possess a steam-engine, or at least that the existence of
such an engine cannot be proved.

I believe that I am able to show that the actual existence of the
transmission of acquired characters cannot be directly proved; that
there are no direct proofs supporting the Lamarckian principle.

If we ask for the facts which can be brought forward by the supporters
of the theory of the transmission of acquired characters, if we inquire
for the observations which induced Darwin, for instance, to adopt such
an hypothesis, or which at least prevented him from rejecting it,—a
very brief answer can be given. There are a small number of
observations made upon man and the higher animals which seem to prove
that injuries or mutilations of the body can, under certain
circumstances, be transmitted to the offspring.

A cow which had accidentally lost its horn, produced a calf with an
abnormal horn; a bull which had accidentally lost its tail, from that
time begat tailless calves: a woman whose thumb had been crushed and
malformed in youth, afterwards had a daughter with a malformed thumb,
and so on.

In a great number of such cases every guarantee for the trustworthiness
of the statements is entirely wanting, and, as His and still earlier
Kant have already said, they are of no greater value as evidence than
the merest tales. But in other cases this assertion cannot be made
without further examination; and a small number of such observations
can indeed claim a scientific investigation and value. I shall
presently discuss this point in greater detail, but I wish now to lay
stress upon the fact that, as far as direct evidence goes, we cannot
bring forward any proofs in favour of the transmission of acquired
characters, except these cases of mutilations. There are no
observations which prove the transmission of functional hypertrophy or
atrophy, and it is hardly to be expected that we shall obtain such
proofs in future, for the cases are not of a kind which lend themselves
to an experimental investigation. The hypothesis that acquired
characters can be transmitted is therefore only directly supported by
the above-mentioned instances of the transmission of mutilations. For
this reason, the defenders of the Lamarckian principle, who have come
forward in rather large numbers recently[296], have endeavoured to show
that these observations are conclusive, and therefore of the highest
importance. For the same reason I believe that it is my duty, as I take
the opposite view, to explain what I think of the value of these
apparent proofs of transmitted mutilations.

It can hardly be doubted that mutilations are acquired characters: they
do not arise from any tendency contained in the germ, but are merely
the reaction of the body under external influences. They are, as I have
recently expressed it, purely somatogenic characters[297], viz.
characters which emanate from the body (_soma_) only, as opposed to the
germ-cells; they are therefore characters which do not arise from the
germ itself.

If mutilations must necessarily be transmitted, or even if they might
occasionally be transmitted, a powerful support would be given to the
Lamarckian principle, and the transmission of functional hypertrophy or
atrophy would thus become highly probable. For this reason it is
absolutely necessary that we should try to come to a definite
conclusion as to whether mutilations can or cannot be transmitted.

We will now consider in greater detail the facts which have hitherto
been brought forward upon this point. It is not my purpose to discuss
every single case which has been mentioned anywhere or by anybody; such
a discussion would hardly lead to any result. I propose to select a
small number of such instances, in order to show why they cannot be
maintained as proofs. I shall chiefly deal with cases which have been
brought forward as especially strong proofs by my opponents, and which
have been carefully and completely examined. I shall attempt to show
that these are not conclusive and that they must be explained in an
entirely different manner. The insufficiency of the proof does not
always depend upon the same circumstances, and, according to the
latter, we may distinguish different classes of cases.

First of all we may briefly mention those instances in which the
necessary precautions have not been taken before drawing conclusions.

To this class belong the tailless cats which were shown at last year’s
(1887) Meeting of the Association of German Naturalists, at Wiesbaden.
These cats had inherited their taillessness, or rather their
rudimentary tails, from the mother cat, which ‘was said’ to have lost
her tail by the wheel of a cart having passed over it. Not only did the
owner of the cats, Dr. Zacharias, consider them as a proof of the
transmission of mutilations, but in a recently-published work, entitled
‘On the Origin of Species, based upon the Transmission of acquired
characters’ (‘Ueber die Entstehung der Arten auf Grundlage des
Vererbens erworbener Eigenschaften’), the author, Prof. Eimer, speaks
of these cats in the preface as a ‘valuable’ instance of the
transmission of mutilations: these examples therefore form part of the
foundation upon which the author builds up his theoretical views.

Certainly, the want of tails in young cats, of which the mother had
lost its tail by an accident, would have been well worth consideration,
but unfortunately there is no trustworthy record as to how the mother
cat became tailless. Without absolute certainty upon this point the
evidence becomes utterly worthless; and Dr. Zacharias has acted very
wisely in afterwards admitting that this is the case, for inherent
taillessness has been known in cats for a long time. The tailless race
of the Isle of Man is mentioned in the first edition of ‘The Origin of
Species’; of course I am referring to Darwin’s work, and not to the
above-mentioned book of the same name, by Prof. Eimer. As to the first
origin of the tailless Manx breed we know no more than about the origin
of that remarkable race of cats with supernumerary toes, which E. B.
Poulton has recently described from Oxford, and has traced through
several generations[298]. These are innate monstrosities which have
arisen from unknown changes in the germ. Similar monstrosities have
been known for a long time, and no one has ever doubted that they can
be transmitted.

It would be equally justifiable to derive the cats with extra toes from
an ancestor of which the toes had been trodden upon, as to derive the
tailless cats of the Isle of Man from an ancestor of which the tail had
been cut off by a cart passing over it, and thus to regard the
existence of the race as a proof of the transmission of mutilations.

But even if it were certain that the tail of the mother cat had been
mutilated, such a fact would not necessarily prove that the rudimentary
tails of the offspring were due to transmission from the mother: they
might have been transmitted from the unknown father. This is probably
not the case with Dr. Zacharias’ cat, for tailless kittens occurred in
several families produced by the same mother; but in other cases the
possibility of the possession of innate taillessness by the father must
be taken into account. The following case is, in this respect, very
instructive.

Last summer, my friend, Prof. Schottelius, of Freiburg, brought me a
kitten with an innate rudimentary tail, which he had accidentally
discovered as one of a family of kittens at Waldkirch, a small town in
the southern part of the Black Forest. The mother of the kitten
possessed a perfectly normal tail; the father could not be identified.

A closer investigation resulted in the following rather unexpected
discovery. For some years past, tailless kittens have frequently
appeared in the families of many different mother cats at Waldkirch,
and this fact is explained in the following manner. A clergyman, who
lived for some time at Waldkirch, had married an English lady who
possessed a tailless male Manx cat. The probability that all the
tailless cats in Waldkirch are more or less distant descendants of that
male cat almost amounts to certainty. Since a male Manx cat has reached
the Black Forest, it might equally well arrive at some other place.

But we will now leave observations such as these, which do not prove
the transmission of a mutilation, because the mutilation itself has not
been established; and we will turn to more serious ‘proofs.’

Let us still consider the tails of domesticated animals. In these
animals a spontaneous and considerable reduction of the tail occurs not
uncommonly, and since the habit of cutting off part of the tail of
young animals prevails in many countries, the coincidence has been
explained as a causal relation, and the question has been raised
whether the disposition towards the spontaneous appearance of
rudimentary tails has not arisen in consequence of the artificial
mutilation practised through many generations. This supposition appears
very plausible at first sight, but the keen scientific criticism of
Döderlein, Richter, and Bonnet, together with careful anatomical
investigations, have shown that, at least in the cases which were
carefully examined, such a causal connection did not exist. It has been
shown that the spontaneous rudimentary tails which occasionally appear
in cats and dogs have an entirely different origin from the
transmission of artificial mutilation. They depend upon an innate
peculiarity of the germ, a peculiarity which is easily and strongly
transmitted. They are monstrosities, like the sixth finger or toe, or,
rather, like the rudimentary fingers and toes, which also occasionally
appear. Bonnet[299] has shown that the rudimentary tails of dogs depend
upon the absence of several vertebrae, together with an abnormal
ossification, and sometimes also with a premature coalescence, of the
vertebrae of the tail.

Bonnet states that in the two first cases examined by him the reduction
occurred at the distal end of the vertebral column in the tail, the
more or less malformed vertebrae being anchylosed. A membranous
appendage extended beyond the end of the reduced caudal vertebrae, as
the so-called ‘soft tail.’ These characters were shown to have been
inherited from the mother and to have undergone progressive development
as regards the number of missing vertebrae and the proportion of
individuals with rudimentary tails.

In a third instance Bonnet found that four to seven of the normal
caudal vertebrae were absent, and that the column in the region of the
tail was characterised by a tendency towards premature anchylosis along
its whole length and not merely in its distal portion. Furthermore the
last three to four vertebrae were distorted and were either placed
transversely to the long axis of the tail, or were so greatly curved
that the tip of the tail was directed forwards.

It is obvious that these changes are not such as we should expect as a
result of the transmission of the mutilation of the tail which is so
commonly practised. If the artificial injury were transmitted we should
not expect that a variable number of the mesial vertebrae would be
absent, but rather those of the tip. There would be no reason why the
existing vertebrae should be degenerate as in the majority of the
caudal vertebrae of the dogs examined by Bonnet.

Entirely similar phenomena have been observed by Döderlein in the
tailless cats which not infrequently occur in Japan. In these cats the
rudimentary vertebrae of the tail were reduced to a short, thin,
inflexible spiral, which formed a knot densely covered with hair on the
posterior part of the animal.

Such a reduction of the tail occurs quite independently of artificial
injury, in individuals of which the parents were not injured: it is
even found in races, such as the dachshund, which, as far as we know,
have never been habitually mutilated.

But the fact is rendered especially interesting because the reduction
of the vertebral column in the region of the tail takes place in very
various degrees. Sometimes only four vertebrae are absent, sometimes as
many as ten. The degree of abnormality in shape and the degree of
coalescence between the vertebrae also differ greatly. Hence Bonnet
rightly concludes that a slow and gradual process of reduction is going
on in these animals, a process which tends, as it were, to shorten the
tail. I intentionally say ‘as it were,’ for of course the statement
must not be taken literally, and we must not conclude that the process
of reduction is a consequence of some hypothetical developmental force
seated in the organism, of which the purpose is to remove the tail. On
the contrary, this instance shows very clearly that the appearance of a
development guided in a certain direction may be produced without the
existence of any motive developmental force.

The disposition of the tail to become rudimentary, in cats and dogs,
may be explained in the simplest way, by the process which I have
formerly called panmixia. The tail is now of hardly any use to these
animals; and neither dog nor cat would perish because they possessed
only an incomplete tail. Hence natural selection does not now exercise
any influence over these parts, and an occasional reduction is no
longer eliminated by the early destruction of its possessor: therefore
such reduction may be transmitted to the offspring.

The race of tailless foxes which, according to Settegast, existed
during the present century on the hunting-grounds of Prince Wilhelm zu
Solms-Braunfels, very soon disappeared; while cats and dogs with
rudimentary tails have been preserved in many cases. Such results are
to be expected, because in these domesticated animals the absence of
the tail did not cause any inferiority in the struggle for existence.

But these facts appear to me to be remarkable in another direction. I
previously mentioned the tailless race of Manx cats. Tradition does not
tell us how it happened that the descendants of the first tailless cat
in the Isle of Man were able to increase and spread in such a manner as
to form the dominant race in the island. But we can easily imagine how
it happened, when we learn that tailless cats are especially
prized[300] in Japan, because people think that they are better
mousers. Every one in Japan wishes to possess a tailless cat, and
people even cut off the tails of normal cats when they cannot obtain
those with congenital rudimentary tails, because they believe that cats
become better mousers in consequence of taillessness. In Waldkirch the
same account of the superiority of tailless cats is curiously enough
also found. We thus see how a slight but striking variation may at once
cause an energetic process of artificial selection, which helps this
variation to predominance: a hint for us to be careful in passing
judgment upon sexual selection, for the latter also works upon such
functionally indifferent but striking variations. In the case of the
cats, man has favoured a particular variation, because the novelty
rather than the beauty of the character surprised and attracted him. He
has attached an imaginary value to the new character, and by artificial
selection has helped it to predominate over the normal form. I see no
reason why the same process should not take place in animals by the
operation of sexual selection.

But now, after this little digression, let us return to the
transmission of mutilations.

We have seen that the rudimentary tails of cats and dogs, as far as
they can be submitted to scientific investigation, do not depend upon
the transmission of artificial mutilation, but upon the spontaneous
appearance of degeneration in the vertebral column of the tail. The
opinion may, however, be still held that the customary artificial
mutilation of the tail, in many races of dogs and cats, has at least
produced a number of rudimentary tails, although, perhaps, not all of
them. It might be maintained that the fact of the spontaneous
appearance of rudimentary tails does not disprove the supposition that
the character may also depend upon the transmission of artificial
mutilation.

Obviously, such a question can only be decided by experiment: not, of
course, experiments upon dogs and cats, as Bonnet rightly remarks, but
experiments upon animals the tails of which are not already in a
process of reduction. Bonnet proposes that the question should be
investigated in white rats or mice, in which the length of the tail is
very uniform, and the occurrence of rudimentary tails is unknown.

Before this suggestion was made, I had already attacked the problem
experimentally. Such a course might, perhaps, have been more natural to
those who maintain the transmission of mutilations, to which I am
opposed. Although I undertook the experiments expecting to obtain
purely negative results, I thought that the latter would not be
entirely valueless; and since the numerous supporters of the
transmission of acquired characters do not seem to be willing to test
their opinion experimentally, I have undertaken the not very large
amount of trouble which is necessary in order to conduct such an
experimental test.

The experiments were conducted upon white mice, and were begun in
October of last year (1887), with seven females and five males. On
October 17 all their tails were cut off, and on November 16 the two
first families were born. Inasmuch as the period of pregnancy is only
22-24 days, these first offspring began to develope at a time when both
parents were without tails. These two families were together eighteen
in number, and every individual possessed a perfectly normal tail, with
a length of 11-12 mm. These young mice, like all those born at later
periods, were removed from the cage, and either killed and preserved,
or made use of for the continuance of the breeding experiments. In the
first cage, containing the twelve mice of the first generation, 333
young were born in fourteen months, viz. until January 16, 1889, and no
one of these had a rudimentary tail or even a tail but slightly shorter
than that of the offspring of unmutilated parents.

It might be urged that the effects of mutilation do not exercise any
influence until after several generations. I therefore removed fifteen
young, born on December 2, 1887, to a second cage, just after they were
able to see, and were covered with hair; their tails were cut off.
These mice produced 237 young from December 2, 1887 to January 16,
1889, every one of which possessed a normal tail.

In the same manner fourteen of the offspring of this second generation
were put in cage No. 3 on May 1, 1888, and their tails were also cut
off. Among their young, 152 in number, which had been produced by
January 16, there was not a single one with an abnormal tail. Precisely
the same result occurred in the fourth generation, which were bred in a
fourth cage and treated in exactly the same manner. This generation
produced 138 young with normal tails from April 23 to January 16.

The experiment was not concluded with the fourth generation; thirteen
mice of the fifth generation were again isolated and their tails were
amputated; by January 16, 1889 they had produced 41 young.

Thus 901 young were produced by five generations of artificially
mutilated parents, and yet there was not a single example of a
rudimentary tail or of any other abnormity in this organ. Exact
measurement proved that there was not even a slight diminution in
length. The tail of a newly-born mouse varies from 10.5 to 12 mm. in
length, and not one of the offspring possessed a tail shorter than 10.5
mm. Furthermore there was no difference in this respect between the
young of the earlier and later generations.

What do these experiments prove? Do they disprove once for all the
opinion that mutilations cannot be transmitted? Certainly not, when
taken alone. If this conclusion were drawn from these experiments alone
and without considering other facts, it might be rightly objected that
the number of generations had been far too small. It might be urged
that it was probable that the hereditary effects of mutilation would
only appear after a greater number of generations had elapsed. They
might not appear by the fifth generation, but perhaps by the sixth,
tenth, twentieth, or hundredth generation.

We cannot say much against this objection, for there are actual
phenomena of variation which must depend upon such a gradual and at
first imperceptible change in the germ-plasm, a change which does not
become visible in the descendants until after the lapse of generations.
The wild pansy does not change at once when planted in garden soil: at
first it remains apparently unchanged, but sooner or later in the
course of generations variations, chiefly in the colour and size of the
flowers, begin to appear: these are propagated by seed and are
therefore the consequence of variations in the germ. The fact that such
variations _never_ occur in the first generation proves that they must
be prepared for by a gradual transformation of the germ-plasm.

It is therefore possible to imagine that the modifying effects of
external influences upon the germ-plasm may be gradual and may increase
in the course of generations, so that visible changes in the body
(_soma_) are not produced until the effects have reached a certain
intensity.

Thus no conclusive theoretical objections can be brought forward
against the supposition that the hereditary transmission of mutilations
requires (e. g.) 1000 generations before it can become visible. We
cannot estimate _a priori_ the strength of the influences which are
capable of changing the germ-plasm, and experience alone can teach us
the number of generations through which they must act before visible
effects are produced.

If therefore mutilations really act upon the germ-plasm as the causes
of variation, the possibility or even probability of the ultimate
appearance of hereditary effects could not be denied.

Hence the experiments on mice, when taken alone, do not constitute a
complete disproof of such a supposition: they would have to be
continued to infinity before we could maintain with certainty that
hereditary transmission cannot take place. But it must be remembered
that all the so-called proofs which have hitherto been brought forward
in favour of the transmission of mutilations assert the transmission of
a single mutilation which at once became visible in the following
generation. Furthermore the mutilation was only inflicted upon one of
the parents, not upon both, as in my experiments with mice. Hence,
contrasted with these experiments, all such ‘proofs’ collapse; they
must all depend upon error.

It is for this reason important to consider those cases of habitual
mutilation which have been continually repeated for numerous
generations of men, and have not produced any hereditary consequences.
With regard to the habitually amputated tails of cats and dogs I have
already shown that there is only an apparently hereditary effect.
Furthermore, the mutilations of certain parts of the human body, as
practised by different nations from times immemorial, have, in not a
single instance, led to the malformation or reduction of the parts in
question. Such hereditary effects have been produced neither by
circumcision[301], nor the removal of the front teeth, nor the boring
of holes in the lips or nose, nor the extraordinary artificial crushing
and crippling of the feet of Chinese women. No child among any of the
nations referred to possesses the slightest trace of these mutilations
when born: they have to be acquired anew in every generation.

Similar cases can be proved to occur among animals. Professor Kühn of
Halle pointed out to me that, for practical reasons, the tail in a
certain race of sheep has been cut off, during the last hundred years,
but that according to Nathusius, a sheep of this race without a tail or
with only a rudimentary tail has never been born. This is all the more
important because there are other races of sheep in which the shortness
of the tail is a distinguishing peculiarity. Thus the nature of the
sheep’s tail does not imply that it cannot disappear.

A very good instance is mentioned by Settegast, although perhaps with
another object in view. The various species of crows possess stiff
bristle-like feathers round the opening of the nostrils and the base of
the beak: these are absent only in the rook. The latter, however,
possesses them when young, but soon after it has left the nest they are
lost and never reappear. The rook digs deep into the earth in searching
for food, and in this way the feathers at the base of the beak are
rubbed off and can never grow again because of the constant digging.
Nevertheless this peculiarity, which has been acquired again and again
from times immemorial, has never led to the appearance of a newly
hatched individual with a bare face.

Thus there is no reason for the assumption that such a result would
occur in the case of the mice even if the experiments had been
continued through hundreds or thousands of generations. The supposition
of the accumulative effect of mutilation is entirely visionary, and
cannot be supported except by the fact that accumulative
transformations of the germ-plasm occur; but of course this fact does
not imply that mutilations belong to those influences which are capable
of changing the germ-plasm. All the ascertained facts point to the
conclusion that they have not this effect. The transmission is all the
more improbable because of the striking form of the mutilation in all
cases which are relied upon as evidence. The only objection which can
be raised is to suppose that the absence of the tail is less easily
transmitted than other mutilations, or that mice possess smaller
hereditary powers than other animals. But there is not the slightest
evidence in favour of either of these suggestions; the supporters of
the Lamarckian principle have, on the contrary, always pointed to the
transmission of mutilated tails as one of their principal lines of
evidence.

The opinion has often been expressed that such transmission need not
occur in every case, but may happen now and then under quite
exceptional conditions with which we are unacquainted: for this reason
it might be urged that all negative experiments and every refutation of
the ‘proofs’ of the transmission of mutilations are not conclusive.
Only recently, a clever young zoologist said in reference to Kant’s
statements upon the subject, that perhaps the most decided opponent of
the transmission of mutilations would not venture nowadays to maintain
his view with such certainty, ‘for it must be admitted that the
transmission of acquired characters may take place at any rate as a
rare exception.’ Similar opinions are often expressed, especially in
conversation, and yet they can mean nothing except that the
transmission of acquired characters has been proved; for if such
transmission can take place at all, it exists, and it does not make the
least difference theoretically whether it occurs in rare cases or more
frequently. Sometimes heredity has been called capricious, and in a
certain sense this is true. Heredity appears to be capricious because
we cannot penetrate into its depths: we cannot predict whether any
peculiar character in the father will reappear in the child, and still
less whether it will reappear in the first, second, or one of the later
children: we cannot predict whether a child will possess the nose of
his father or mother or one of the grandparents. But this certainly
does not imply that the results are due to chance: no one has the right
to doubt that everything is brought about by the operation of certain
laws, and that, with the fertilization of the egg, the shape of the
nose of the future child has been determined. The co-operation of the
two tendencies of development contained in the two conjugating
germ-cells produces of necessity a certain form of nose. The observed
facts enable us to know something of the laws under which such events
take place. Thus, for instance, among a large number of children of the
same parents some will always have the form of the nose of the mother
or of the mother’s family; others will have the nose of the father’s
family, and so on.

If we apply this argument to the supposed transmission of mutilations,
such transmission, if possible at all, must occur a certain number of
times in a certain number of cases: it must occur more readily when
both parents are mutilated in the same way, or when the mutilation has
been repeated in many generations, etc. It is extremely improbable that
it would suddenly occur in a case where it was least expected, while it
did not occur in 900 cases of the most favourable kind. Those who
recognise in the doubtful cases of transmission of a single mutilation
present in only one of the parents, proofs of the existence of the
disputed operation of heredity, quite forget that the transmission
presupposes a very marvellous and extremely complex apparatus which if
present at all ought, under certain conditions, to become manifest
regularly, and not only in extremely exceptional cases. Nature does not
create complex mechanisms in order to leave them unused: they exist by
use and for use. We can readily imagine how complex the apparatus for
the transmission of mutilations or acquired characters generally must
be, as I have tried to show in another place. The transmission of a
scar to the offspring e. g. presupposes first of all that each
mechanical alteration of the body (_soma_) produces an alteration in
the germ-cells: this alteration cannot consist in mere differences of
nutrition, only affecting an increased or decreased growth of the
cells: it must be of such a kind that the molecular structure of the
germ-plasm would be changed. But such a change could not in the least
resemble that which occurred at the periphery of the body in the
formation of the scar: for there is neither skin nor the preformed germ
of any of the adult organs in the germ-plasm, but only a uniform
molecular structure which, in the course of many thousand stages of
transformation, must tend to the formation of a soma including a skin.
The change in the germ-plasm which would lead to the transmission of
the scar, must therefore be of such a kind as to influence the course
of ontogeny in one of its later stages, so that an interruption of the
normal formation of skin, and the intercalation of the tissue of the
scar, would occur at a certain part of the body. I do not maintain that
equally minute changes of the germ-plasm could not occur: on the
contrary, individual variation shows us that the germ-plasm contains
potentially all the minutest peculiarities of the individual; but I
have in vain tried to understand how such minute changes of the
germ-plasm in the germ-cells could be caused by the appearance of a
scar or some other mutilation of the body. In this respect I think that
Blumenbach’s condition is nearly fulfilled: he was inclined to declare
himself against the transmission of mutilations, but only if it were
proved that such transmission was _impossible_. Although this cannot be
strictly proved, it can nevertheless be shown that the apparatus
presupposed by such transmission must be so immensely complex, nay! so
altogether inconceivable, that we are quite justified in doubting the
possibility of its existence as long as there are no facts which prove
that it _must_ be present. I therefore do not agree with the recent
assertion[302] that Blumenbach’s condition cannot be fulfilled to-day,
just as it was impossible at the time when it was first brought
forward. But if nevertheless such a mysterious mechanism existed
between the parts of the body and the germ-cells, by means of which
each change in the former could be reproduced in a different manner in
the latter, the effects of this marvellous mechanism would certainly be
perceptible and could be subjected to experiment.

But at present we have no evidence of the existence of any such
effects; and the experiments described above disprove all the cases of
the supposed transmission of single mutilations.

Of course, I do not maintain that such cases are to be always explained
by want of sufficient observation. In order to make my position clear,
I propose to discuss two further classes of observations. First of all,
there are very many cases of the apparent transmission of mutilations
in which it was not the mutilation or its consequences which was
transmitted, but the predisposition of the part in question to become
diseased. Richter[303] has recently pointed out that arrests of
development, so slight as to be externally invisible, frequently occur,
and that such arrests exhibit a tendency to lead to the visible
degeneration of parts in which they occur, as the result of slight
injuries. Since therefore the predisposition towards such arrest is
transmitted by the germ—occasionally even in an increased degree—the
appearance of a transmitted injury may arise. In this way Richter
explains, for instance, the frequently quoted case of the soldier who
lost his left eye by inflammation fifteen years before he was married,
and who had two sons with left eyes malformed (microphthalmic).
Microphthalmia is an arrest of development. The soldier did not lose
his eye simply because it was injured, but because it was predisposed
to become diseased from the beginning and readily became inflamed after
a slight injury. He did not transmit to his sons the injury or its
results, but only microphthalmia, the predisposition towards which was
already innate in him, but which led in his sons from the beginning,
and without any obvious external injury, to the malformation of the
eye. I am inclined to explain the case which Darwin in a similar manner
adduced, during the later years of his life, in favour of the
transmission of acquired characters, and which seemed to prove that a
malformation of the thumb produced by chilblains can be transmitted.
The skin of a boy’s thumbs had been badly broken by chilblains
associated with some skin disease. The thumbs became greatly swollen
and remained in this state for a long time; when healed they were
malformed, and the nails always remained unusually narrow, short, and
thick. When this man married and had a family, two of his children had
similarly malformed thumbs, and even in the next generation two
daughters had malformed thumbs on both hands. The case is too
imperfectly known to admit of adequate criticism; but one may perhaps
suggest that the skin of different individuals varies immensely in its
susceptibility to the effects of cold, and that many children have
chilblains readily and badly, while others are not affected in this
way. Sometimes members of the same family vary in this respect, and the
greater or less predisposition towards the formation of chilblains
corresponds with a different constitution of the skin, in which some
children follow the father and others follow the mother. In Darwin’s
instance a high degree of susceptibility of the skin of the thumb was
obviously innate in the father, and this susceptibility was certainly
transmitted, and led to the similar malformation of the thumbs of the
children, perhaps very early and after the effect of a comparatively
slight degree of cold[304].

The last class of cases which I should wish to consider, refer to
observations in which the mutilation of the parent was certain, and in
which a malformation similar to the mutilation had appeared in the
child, but in which exact investigation shows that the malformations in
parent and child do not in reality correspond to each other.

In this class I include an instance which has only become known during
the present year (1888), and which has been observed as exactly as
possible by an anthropologist and physician, who has worked up the
history of the case. Dr. Emil Schmidt communicated to this year’s
meeting of the German Anthropologists’ Association at Bonn a case which
indeed seems at first sight to prove that mutilations of the human ear
can be transmitted. As Dr. Schmidt has been kind enough to place at my
disposal all the material which he collected upon the subject, I have
been able to examine it more minutely than is generally possible in
such cases; and I will discuss it in detail, as it seems to me to be of
fundamental importance in the history of human errors upon this subject.

In a most respectable and thoroughly trustworthy family, the mother
possesses a cleft ear-lobe upon one side. She quite distinctly
remembers that when playing, between the ages of six and ten years,
another child tore out her ear-ring, and that the wound healed so that
the cleft remained. Later on a new hole was made in the posterior part
of the lobe. She had seven children, and the second of these, who is
now a full-grown man, has a cleft ear-lobe on the same side as the
mother. It is not known whether the mother possessed an innate
malformation of the ear before it was mutilated, but, judging from the
present appearance of the ear, this is extremely improbable.
Furthermore, the existence of an innate cleft in the ear-lobe has never
been previously observed. The parents of the mother did not possess any
malformation of the ear. The conclusion seemed to be therefore
inevitable that the transmission of an artificial cleft in the ear-lobe
had really taken place.

But we must not be too hasty in forming an opinion. When we compare the
figures I. and II., representing the two ears, we are first of all
struck by the fact that the malformation of the ear of the son has an
entirely different appearance from that of the mother. The ear-lobe of
the latter is quite normally formed; it is broad and well-developed,
and only exhibits a median vertical furrow which is the result of the
mutilation. The ear-lobe of the son, on the other hand, is extremely
minute, one might even maintain that it is completely wanting. In my
opinion a cleft is not present at all, but the far higher posterior
corner of the ear forms the end of its posterior margin—the so-called
helix. But even if another opinion were pronounced with regard to the
interpretation of this part, there is one other circumstance to be
taken into account, which appears to me to be absolutely conclusive,
and which completely excludes the interpretation of this malformation
as the transmission of a mutilation.

[Illustration: FIG. I. _H._ Helix. _Cr. Ah._ Crura anthelicis._ Ah._
Anthelix. _Cch._ Concha. _Hl^1_. and _Hl^2_. Holes 1 and 2 for
ear-rings. _Lob._ Ear-lobe. _Sp. H._ Spina helicis. _Inc._ Incisura
intertragica. _Tr._ Tragus. _Atr._ Antitragus.]

If we compare the ears with each other, that of the mother with that of
the son, not only the anatomist but every trained observer will at once
be struck by the fact that they are totally different in their outlines
as well as in every detail. The upper margin of the ear is very broad
in the mother, in the son it is quite pointed: the so-called _crura
anthelicis_ are normally developed in the mother, in the son they can
hardly be distinguished and open in an anterior direction, while in the
mother they are directed upwards. The concha itself, the _incisura
intertragica_, in short everything in the two ears, is as different as
it can possibly be in the ears of two individuals.

But this fact obviously proves that the son does not possess the ear of
his mother, but probably that of his father or grandfather.
Unfortunately the father and grandfather have been now dead for a long
time, so that we cannot obtain certain evidence upon this point. At all
events, the son does not possess the ear of his mother, and it would be
very rash to suppose that he has inherited the ear from the father, but
the malformation of the ear-lobe from the mother—a malformation which,
as it seems to me, is certainly quite different from that of his
mother’s ear. I said that this case was of fundamental importance
chiefly because it shows very distinctly, on the one hand, how
difficult it is to bring together the material which is absolutely
necessary for the correct understanding of a single case, and on the
other hand, how carefully the abnormalities must be compared and
examined if we wish to escape wrong conclusions. Such precautions have
hitherto been rarely taken with the necessary accuracy; people are in
most cases satisfied with the knowledge that an abnormality is present
in the child on the same part which had been malformed by mutilation in
the parent.

But if we are to speak of the transmission of a mutilation, it must be
shown, before everything else, that the malformation of the child
corresponds precisely to the mutilation of the parent.

For this reason the older observations upon this subject are, in most
cases, entirely valueless.

The readiness with which we may be deceived is shown by the fact that I
myself nearly became a victim during the past year (1888). A friend of
mine, in order to convince me of the transmission of mutilations,
called my attention to a linear scar on his left ear, which extended
from the upper margin of the helix for some distance upon the posterior
part of the anthelix, giving it the appearance of a small, rather sharp
ridge. The scar had been caused by a cut from a duelling sword, which
the gentleman had received during his residence at the University.
Strangely enough, the left ear of his daughter, who is five years old,
exhibits a similar peculiarity. The posterior part of the anthelix
forms a rather sharp and narrow ridge like that of the father, although
the scar itself is wanting.

I must admit that I was at first rather puzzled by this fact, but the
mystery was soon solved in a very simple manner. I asked the father to
show me his right ear, and I then saw that this ear possessed a similar
ridge on the posterior part of the anthelix. Only the scar was absent,
which in the left ear brought the crest of the ridge into still greater
prominence. The ridge was therefore only an individual peculiarity in
the formation of the ear of the father,—a peculiarity which had been
transmitted to one ear of the child. No transmission of the mutilation
had taken place.

In the same manner, many of the so-called proofs of the transmission of
mutilations would be shown, by a careful examination, to be deceptive.
We must not expect to succeed in all of them, for in most cases the
investigation cannot be completed, chiefly because the condition of the
part in question in the ancestors is not known or is only known in an
insufficient manner. This is the reason why fresh examples of such
so-called proofs continue to appear from time to time,—proofs which do
not admit of a searching criticism because something, and in most cases
very much, is invariably wanting. But it will be admitted that even a
very large number of incomplete proofs do not make a single complete
one. On the other hand, it may be asserted that a single instance of
coincidence between a mutilation in the parent and a malformation in
the offspring, even if well established, would not constitute a proof
of the transmission of mutilations. Not every _post hoc_ is also a
_propter hoc_. Nothing illustrates this better than a comparison
between the ‘proofs’ which are even now brought forward in favour of
the transmission of mutilations and the ‘proofs’ which supported the
belief in the efficacy of so-called ‘maternal impressions’ during
pregnancy, a belief which was universally maintained up to the middle
of the present century. Many of those ‘proofs’ were simply old wives’
fables, and were based upon all kinds of subsequent inventions and
alterations. But it cannot be denied that there are a few undoubtedly
genuine observations upon cases in which some character in the child
reminds us in a striking manner of a deep psychical impression by which
the mother was strongly affected during pregnancy.

Thus a trustworthy person told me of the following case. A well-known
medical authority cut his leg above the ankle with a knife: his wife
was present at the time and was much frightened. She was then in the
third month of pregnancy: the child when born was found to have an
unusual mark upon the same place above the ankle. People almost forget
nowadays the tenacity with which the idea of maternal impressions was
kept up until the middle of this century; but it is only necessary to
read the received German text-book on physiology of fifty years ago,
viz. that of Burdach, in order to be convinced of the accuracy of this
statement. Not only does Burdach give a number of ‘conclusive’ cases in
man and even in animals (cows and deer), but he also attempts to
construct a theoretical explanation of the supposed process. This is
undertaken in the following manner,—‘Imagination influences the
function of organs;’ but the function of the embryo is the ‘tendency
towards development, and hence the influence [of maternal imagination]
can make itself felt only as variations in the mode of development.’
Thus by exchanging the conception of function for that of the
development of organs, Burdach comes to the conclusion that ‘homologous
organs of the mother and the embryo are in such connexion’ that when
the former are disturbed a corresponding ‘change in the formation of
the latter may arise.’

It seems to be not without value for the appreciation of the questions
with which we are dealing to remember that the idea of ‘maternal
impressions’ was only comparatively recently believed to be a
scientific theory, and that the proofs in support of it were brought
forward in form and language as scientific proofs. In Burdach’s book we
even meet with detailed ‘proofs’ that violent mental shocks produced by
maternal impressions may not only exercise their influence upon one but
even upon several children born successively, although with diminishing
strength. ‘A young wife received a shock during her first pregnancy
upon seeing a child with a hare-lip, and she was constantly haunted
with the idea that her child might have the same malformation. She was
delivered of a child with a typical hare-lip: her next child had an
upper lip with a less-marked cleft; while the third possessed a red
mark instead of a cleft.’

Now what can be said about such ‘proofs’? We may probably rightly
conjecture that Burdach, who was in other respects a clever
physiologist, was in this subject somewhat credulous: but there are
also instances about which there is not the slightest doubt. I may
remind the reader of a case which has been told by no other than the
celebrated embryologist, Carl Ernst von Baer[305].

‘A lady was very much upset by a fire, which was visible at a distance,
because she believed that it was in her native place. As the latter was
seven German miles distant, the impression had lasted a long time
before it was possible to receive any certain intelligence, and this
long delay affected the mind of the lady so greatly, that for some time
afterwards she said that she constantly saw the flames before her eyes.
Two or three months afterwards she was delivered of a daughter who had
a red patch on the forehead in the form of a flame. This patch did not
disappear until the child was seven years old.’ Von Baer added, ‘I
mention this case because I am well acquainted with it, for the lady
was my own sister, and because she complained of seeing flames before
her eyes before the birth of the child, and did not invent it
afterwards as the “cause” of the strange appearance.’

Here then we have a case which is absolutely certain. Von Baer’s name
is a guarantee for absolute accuracy. Why then has science, in spite of
this, rejected the whole idea of the efficacy of ‘maternal impressions’
ever since the appearance of the treatises by Bergmann and
Leuckart[306]?

Science has rejected this idea for many and conclusive reasons, all of
which I am not going to repeat here. In the first place, because our
maturer knowledge of the physiology of the body shows that such a
causal connexion between the peculiar characters of the child and, if I
may say so, the corresponding psychical impressions of the mother, is a
supposition which cannot be admitted; but also and chiefly because a
single coincidence of an idea of the mother with an abnormality in the
child does not form the proof of a causal connexion between the two
phenomena.

I do not doubt that among the many thousands of present and past
students in German Universities, whose faces are covered with scars,
there may be one with a son who exhibits a birth-mark on the spot where
the father possesses a scar. All sorts of birth-marks occur, and why
should they not sometimes have the appearance of a scar? Such a case,
if it occurred, would be acceptable to the adherents of the theory of
the transmission of acquired characters; it would in their opinion
completely upset the views of their opponents.

But how could such a case, if it were really established, be capable of
proving the supposed form of hereditary transmission, any more than von
Baer’s case could prove the theory of the efficacy of ‘maternal
impressions’?

I am of opinion that the extraordinary rarity of such cases strongly
enforces the fact that we have to do with an accidental and not a
causal coincidence. If scars could be really transmitted, we should
expect very frequently to find birth-marks which correspond to scars
upon the face of the father,—viz. in almost all cases in which the son
had inherited the type of face possessed by the father. If this were so
we should have to be seriously concerned about the beauty of the next
generation in Germany, as so many of our undergraduates follow the
fashion of decorating their faces with as many of these ‘honourable
scars’ as possible.

I have spoken of ‘maternal impressions’ because I wished to show that,
until quite recently, distinguished and acute scientific men have
adhered to an idea, and believed that they possessed the proof of an
idea, which has now been completely and for ever abandoned by science.
But in addition to this, there is a very close connexion between the
theory of the efficacy of maternal impressions and that of the
transmission of acquired characters, and sometimes they are even
confounded together.

Last year a popular scientific journal quoted the following case as a
proof of the transmission of mutilations. I do not, however, wish to
imply that the editor must be held responsible for the errors of a
correspondent. ‘In November, 1864, a pregnant merino sheep broke its
right fore-leg, about two inches above the knee-joint; the limb was put
in splints and healed a long time before the following March, when the
animal produced young. The lamb possessed a ring of black wool from two
to three inches in breadth round the place at which the mother’s leg
had been broken, and upon the same leg.’ Now if we even admitted that a
ring of black wool could be looked upon as a character which
corresponds to the fracture of the mother’s leg, the case could not
possibly be interpreted as the transmission of a mutilation, but as an
instance of the efficacy of maternal impressions; for the ewe was
already pregnant when she fractured her leg. The present state of
biological science teaches us that, with the fusion of egg and
sperm-cell, potential heredity is determined[307]. Such fusion
determines the future fate of the egg-cell and the individual with all
its various tendencies.

Such tales, when quoted as ‘remarkable facts which prove the
transmission of mutilations,’ thoroughly deserve the contempt with
which they have been received by Kant and His. When the above-mentioned
instance was told me, I replied, ‘It is a pity that the black wool was
not arranged in the form of the inscription “To the memory of the
fractured leg of my dear mother.”’

The tales of the efficacy of ‘maternal impressions’ and of the
transmission of mutilations are closely connected, and break down
before the present state of biological science. No one can be prevented
from believing such things, but they have no right to be looked upon as
scientific facts or even as scientific questions. The first was
abandoned in the middle of the present century, and the second may be
given up now; when once discarded we need not fear that it will ever
again be resuscitated.

It is hardly necessary to say that the question as to the transmission
of acquired characters is not completely decided by the unconditional
rejection of the transmission of mutilations. Although I am of opinion
that such transmission does not take place, and that we can explain the
phenomena presented by the transformation of species without this
supposition, I am far from believing that the question is settled,
simply because the transmission of mutilations may be dismissed into
the domain of fable. But at all events we have gained this much,—that
the only facts which appear to directly prove a transmission of
acquired characters have been refuted, and that the only firm
foundation on which this hypothesis has been hitherto based has been
destroyed. We shall not be obliged, in future, to trouble about every
single so-called proof of the transmission of mutilations, and
investigation may be concentrated upon the domain in which lies the
true decision as to the Lamarckian principle, it may be concerned with
the explanation of the observed phenomena of transformation.

If, as I believe, these phenomena can be explained without the
Lamarckian principle, we have no right to assume a form of transmission
of which we cannot prove the existence. Only if it could be shown that
we cannot now or ever dispense with the principle, should we be
justified in accepting it. I do not think that I can represent the
state of the subject better than by again referring to the metaphor of
the ship. We see it moving along with all sails set, we can discern the
presence of neither paddles nor screw, and as far as we can judge there
is no funnel, nor any other sign of an engine. In such a case we shall
not be justified in concluding that an engine is present and has some
share in the movement of the vessel, unless the movement is of such a
kind that it is impossible to explain it as due to the unaided action
of the wind, the current, and the rudder. Only if the phenomena
presented by the progress of organic evolution are proved to be
inexplicable without the hypothesis of the transmission of acquired
characters, shall we be justified in retaining such an hypothesis.

------------------------------------------------------------------------

                       Footnotes for Essay VIII.

Footnote 293:

  It is true that he based his opinions upon entirely erroneous
  theories as to the constancy of species. Compare Brock, ‘Einige
  ältere Autoren über die Vererbung erworbener Eigenschaften’ in
  ‘Biolog. Centralblatt,’ Bd. VIII, p. 491 (1888): see also Hugo
  Spitzer, ‘Beiträge zur Descendenz-theorie und zur Methodologie der
  Naturwissenschaft,’ Leipzig, 1886, pp. 515 et seq.

Footnote 294:

  W. His, ‘Unsere Körperform,’ Leipzig, 1875.

Footnote 295:

  See Essay II in the present volume.

Footnote 296:

  [One of the most remarkable forms of this revival of Lamarckism is
  the establishment in America of a ‘Neo-Lamarckian School,’ which
  includes among its members many of the most distinguished American
  biologists. One of the arguments upon which the founders of the
  school have chiefly relied is derived from the comparative morphology
  of mammalian teeth. The evolution of the various types are believed
  to be due to modifications in shape, produced by the action of
  mechanical forces (pressure and friction) during the life of the
  individual. The accumulation of such modifications by means of
  heredity explains the forms of existing teeth.

  It may be reasonably objected that the most elementary facts
  concerning the development of teeth prove that their shapes cannot be
  altered during the lifetime of the individual, except by being worn
  away. The shape is predetermined before the tooth has cut the gum.
  Hence the Neo-Lamarckian School assumes, not the transmission of
  acquired characters, but the transmission of characters which the
  parent is unable to acquire!—E. B. P.]

Footnote 297:

  See p. 412 of the preceding Essay (VII).

Footnote 298:

  [See ‘Nature,’ vol. xxix. p. 20, and vol. xxxv. p. 38. In the latter
  article nine generations are recorded, and in both articles figures
  of the normal and abnormal feet are given. Additional generations and
  many more families have been since observed, and an account of these
  observations will shortly be published in the same paper. The breed
  originally came from Bristol. In the observations recorded, the
  abnormality of the offspring is an indication of the hereditary
  strength of the female parents, while the degree of normality is a
  similar test of heredity through the male parents; for the female
  parents were always abnormal, the male parents always normal. The
  most abnormal kitten observed possessed seven toes on each forefoot,
  seven toes on the right hind foot (three more than the normal
  number), and six on the left hind foot. Kittens with seven toes on
  the forefeet and six on the hind were comparatively common, and all
  intermediate conditions between this and the normal were of frequent
  occurrence. Cats with extra toes are, I think, not uncommon in most
  countries, and the fact that the peculiarity is transmitted is also
  well known. The object of the investigation alluded to was to observe
  the transmission systematically through many generations.—E. B. P.]

Footnote 299:

   Bonnet, ‘Die stummelschwänzigen Hunde im Hinblick auf die Vererbung
  erworbener Eigenschaften,’ Anat. Anzeiger, Bd. III, 1888, p. 584; see
  also ‘Beiträge zur patholog. Anatomie und allgem. Pathologie’ by
  Ziegler and Nauwerck, Bd. IV, 1888.

Footnote 300:

  See the interesting remarks by Döderlein on this point, which Dr.
  Ischikawa of Japan tells me are quite correct. Döderlein, ‘Ueber
  schwanzlose Katzen,’ Zool. Anzeiger, vol. x. Nov. 1887, No. 265.

Footnote 301:

  It is certainly true that among nations which practise circumcision
  as a ritual, children are sometimes born with a rudimentary prepuce,
  but this does not occur more frequently than in other nations in
  which circumcision is not performed. Rather extensive statistical
  investigations have led to this result.

Footnote 302:

  See Brock, ‘Biolog. Centralblatt,’ Bd. VIII. p. 497, 1888.

Footnote 303:

  W. Richter, ‘Zur Vererbung erworbener Charaktere,’ Biolog.
  Centralblatt, Bd. VIII. 1888, p. 289.

Footnote 304:

  This case was not observed by Darwin himself, but was communicated to
  him by J. P. Bishop of Perry, in North America (see ‘Kosmos,’ vol.
  ix. p. 458). Quite apart from the fact that it is by no means certain
  whether the father did not already possess an innate malformation of
  the thumb, exact data are wanting as to the time during which the
  thumb was diseased, and as to the time when the malformation of the
  thumb was first observed in the children and the grandchildren;
  whether at birth or at a later period. For a thorough criticism it
  would also be necessary to have figures of the thumbs. I should not
  have alluded to this case, because of its incomplete history, if it
  had not appeared to me to illustrate the ideas mentioned above. Of
  course I do not maintain that I have suggested the right explanation
  in this particular case. It is possible that the father possessed an
  inherent malformation of the thumb which he had forgotten by the time
  that he came to have children and grandchildren, and was struck by
  the abnormality of their thumbs.

Footnote 305:

  See Burdach, ‘Lehrbuch der Physiologie,’ Bd. II, 1835-40, p. 128.

Footnote 306:

  See Handwörterbuch der Physiologie von Rud. Wagner, Artikel
  ‘Zeugung,’ von Rud. Leuckart.

Footnote 307:

  See V. Hensen, ‘Physiologie der Zeugung.’ Leipzig, 1881.

------------------------------------------------------------------------




                                 INDEX.


  Abutilon, polymorphic flowers of, pp. 320, 323.
  Acanthia lectularia, length of life of, 42.
  Acineta, 151.
  Acquired characters, meaning of, 169;
    on supposed botanical proofs of transmission of, 390, 397.
  Acridium migratorium, length of life of, 40.
  Actinia mesembryanthemum, length of life of, 54.
  Actinosphaerium, 117, 118.
  Activity and length of life, 7, 8.
  Adansonia, length of life of, 6.
  Adler, on the formation of galls, 302.
  After-effects, 403.
  Aglia tau, deposition of eggs, 18;
    length of life of, 18, 59.
  Algae, immortality of unicellular, 25.
  Amoeba, length of time of fission of, 8;
    immortality of, 25;
    fission of, 25, 64.
  Amphibia, polar bodies of, 340, 352.
  Amphileptus meleagris, fission of, 148.
  Amphorina coerulea, polar bodies of, 189.
  Anabiosis, 25, 38.
  Ancylus, length of life of, 56.
  Andricus, length of life of summer generation, 50.
  Anisotropism, 400.
  Anlagen, 192.
  Anodonta, length of life of, 56, 57.
  Ants, duration of life of male and female, 18, 48, 50, 51, 52, 59,
     156.
  Aphilotrix, length of life of imago of, 50.
  Aphis, length of life of, 41;
    parthenogenesis of, 228, 289;
    polar bodies of, 349.
  Apis, _see_ Bees.
  Apus, 152, 324.
  Ascaris, 133, 144;
    fertilization of, 177;
    nuclear division in ovum of, 188, 232, 360;
    spermatogonia of, 220;
    spermatogenesis of, 375.
  Ascidians, length of life of, 57.
  Atavism, 179.
  Atrophy, of organs, 85, 86.
  Auerbach, on fertilization, 355.

  Bacteria, in dead Cockchafer, 46.
  Baer, von, 194;
    on the influence of maternal impressions on the offspring, 445.
  Balanus, polar bodies of, 218.
  Balbiani, on nuclear division, 187;
    on pole-cells, 197;
    on origin of ova, 222.
  Balfour, on impregnation, 175;
    on polar bodies, 214, 225, 339, 345, 353.
  Bear, length of life of, 13.
  Bees, length of larval life of, 15;
    length of life of queen of, 18, 52, 156;
    of drones, 18, 53; of workers, 52, 59, 156;
    activity of, 48;
    oviposition of, 52, 54;
    death of male, 63, 120, 132;
    loss of limbs in development of larva, 89;
    nuptial flight of, 93;
    development of eggs of, 226, 234, 285, 351.
  Begonia, propagation of, 211.
  Beneden, van, on fertilization in Ascaris, 177, 188, 355, 360;
    on polar bodies, 214, 340, 345, 353;
    on Polkörperchen, 216;
    on spermatogonia in Ascaris, 220, 375;
    on nuclear division, 231;
    on sexual reproduction, 282.
  Berthold, on male parthenogenesis, 247.
  Bessels, on importance of fertilization, 235.
  Beyerinck, on the formation of galls, 302.
  Biorhiza, length of life of imago of, 50.
  Birds, length of life of, 11, 36;
    factors in duration of life of, 12.
  Blackbird, length of life of, 6, 11, 36.
  Blaps, length of life of imago of, 47, 48.
  Blastogenic characters, 412.
  Blochmann, on polar bodies, 349.
  Blow-flies, length of larval life of, 15.
  Boar, length of life of, 14.
  Bombinator igneus, nature of ovum of, 125.
  Bombus, 53.
  Bombyces, flight of females impeded by eggs, 17;
    habits of, 44.
  Bonellia viridis, unequal length of life of male and female, 59.
  Bonnet, on rudimentary tails in dogs, 428.
  Born, on position of nucleus in ova, 177;
    on double impregnation, 382.
  Bosmina, parthogenesis of, 325.
  Brooks, on heredity, 166, 326.
  Brown-Séquard, experiments on guinea-pigs, 81, 310, 313.
  Bulimus, length of life of, 55.
  Buprestis splendens, length of life of, 47.
  Burdach, on the influence of maternal impressions on the offspring,
     444.
  Bütschli, on polar bodies, 188, 214, 224, 340;
    on sexual reproduction, 282;
    on processes of fertilization, 355.
  Butterflies, climatic varieties of, 99;
    death of, 120.
  Bythotrephes, spermatozoa of, 176;
    summer eggs of, 239;
    winter eggs of, 348.

  Calberla, on impregnation in Petromyzon, 175.
  Canary birds, length of life of, 36;
    plumage of, 321.
  Carabus auratus, length of life of imago of, 47.
  Carnoy, on karyokinesis in ovum of Ascaris, 360, 368;
    on spermatogenesis, 375.
  Carp, length of life of, 6.
  Cat, length of life of, 6.
  Catallacta, 123.
  Caterpillars, length of life of phytophagous, 15.
  Cells, renewal of, 21;
    nourishment of, 29;
    death of, 59.
  Cephalopods, length of life of, 56.
  Cerambyx heros, length of life of imago of, 47.
  Cetochilus, polar bodies of, 218.
  Chermes, parthogenesis of, 294;
    galls of, 401.
  Cherry-tree, in Ceylon, 406.
  Chrysomela varians, ovoviviparous development of, 48, 49.
  Chydorus, parthenogenesis of, 325.
  Cicada, length of life of, 41, 42.
  Cienkowsky, on conjugation, 286.
  Cionea intestinalis, length of life of, 57.
  Circumcision, 434.
  Cirrhipedes, complementary males of, 58.
  Clausilia, length of life of, 55.
  Cockchafer, length of larval life of, 16;
    length of imaginal life of, 46.
  Coleoptera, length of life of, 46.
  Colpoda cucullus, fission of, 148.
  Conjugation, 282, 286.
  Continuity of germ-plasm, 104.
  Copepods, unequal length of life in the two sexes of parasitic, 58.
  Coryne, origin of sexual bud, 205.
  Cossus ligniperda, length of larval life of, 15.
  Crayfish, length of life of, 6.
  Cuckoo, length of life of, 11, 36.
  Cyclas, length of life of, 56.
  Cynipidae, length of life of, 49;
    number of eggs of agamic, 50;
    deposition of eggs of, 93;
    number of males of, 293.
  Cynips, amount of nuclear matter in egg of, 229;
    parthenogenesis of, 274, 290, 293.
  Cypris, 294.
  Cyto-idioplasm, 181, 184.
  Cytoplasm, 184.

  Daphnidae, segmentation of the egg of, 73, 199;
    loss of jaws in development of, 89;
    winter eggs of, 121;
    sperm cells of, 175, 176;
    parthenogenesis of, 228, 325;
    summer eggs of, 236, 239, 240;
    polar bodies in parthenogenetic eggs of, 249, 345, 350.
  Darwin, on constancy of number of individuals in successive
     generations, 12;
    on Pangenesis, 77, 370;
    on atrophy of organs, 85, 90;
    on cross fertilization, 309;
    on effect of external influences, 391, 423.
  Daucus, structure of root altered by cultivation, 414.
  Death, origin of, 20, 21, 143;
    relation to reproduction, 21, 120, 132, 154;
    necessity of, 23, 24, 134, 159;
    utility of, 24, 112, 135, 153;
    an adaptation produced by natural selection, 24, 28, 60;
    not universal, 25, 27, 111, 119;
    by sudden shock, 63;
    meaning of, 113;
    definition of, 114;
    of the soma, 154.
  Degeneration of organs, _see_ Atrophy.
  Detmer, on transmission of acquired characters in plants, 390.
  Development amongst Protozoa, 149.
  Diatomaceae, fission of, 65.
  Dicyemids, 131, 141.
  Diptera, length of life of, 42;
    pole cells of, 197, 206, 210, 216;
    as fertilizers of flowers, 309.
  Döderlein, on tailless cats, 390, 428, 430.
  Dragon-flies, length of larval life of, 15;
    length of life of imago, 17, 40.
  Dryophanta, length of life of summer generation of, 49;
    of winter generation, 50.
  du Bois Reymond, on the transmission of acquired characters, 82, 390,
     422.
  Duration of life governed by needs of species, 9.
  Düsing, on origin of sex, 239, 241.

  Eagles, length of life of, 11, 37;
    weight of, 14.
  Echinodermata, origin of germ-cells, 202.
  Echinus, polar bodies of, 351.
  Ectocarpus, male parthenogenesis in, 247.
  Eggs, number laid by various birds, 12, 37;
    of insects, 17.
  Eider-ducks, length of life of, 11.
  Eimer, on the inheritance of mutilations, 426.
  Eleodes grandis, and dentiper, length of life of imago of, 47.
  Elephants, length of life of, 6;
    gestation of, 7.
  Encystment, relation to death of, 112, 115, 116, 120, 158;
    protective, 117, 121;
    of Rhizopoda, 121.
  Entoniscidae, unequal length of life in male and female, 58.
  Ephemeridae, length of life of imago of, 40, 156.
  Epigenesis, theory of, 316.
  Eristalis tenax, length of life of, 43.
  Estheridae, 228.
  Euglypha, identity of products of fission of, 26, 64, 65.
  Eupithecia, length of life of, 45.

  Falcons, length of life of, 11, 37.
  Fiedler, on polar bodies in sponges, 217.
  Flemming, on nuclear division, 187, 231, 359, 361.
  Flourens, on length of life, 7.
  Fol, on fusion of nuclei, 174, 189;
    on origin of ova, 222;
    on multiple impregnation, 236, 238, 382;
    on polar bodies, 340, 351;
    on process of fertilization, 355.
  Formica sanguinea and fusca, length of life of, 51.
  Fox, length of life of, 14.

  Galls, 302, 401.
  Galton, on transfusion in Rabbits, 166;
    on heredity, 172;
    on twins, 380.
  Gannets, numbers collected each year, 37.
  Geotropism, 398.
  Germ, meaning of, 148.
  Germ-cells, 73;
    predisposition of the, 84, 102;
    fluctuations in, 102;
    not continuous, 173.
  Germ-plasm, 80, 191, 266, 341, 357, 371, 403;
    continuity of, 104, 168, 173, 184;
    definition of, 174.
  Goliathus cacicus, length of life of imago of, 47.
  Goose, length of life of the wild, 37.
  Götte, on necessity of death, 112;
    on rejuvenescence, 115, 124;
    on death of Metazoa, 125.
  Gregarines, 148, 149, 202.
  Grobben, on polar bodies of Cetochilus, 218.
  Gruber, on regeneration amongst Infusoria, 185.
  Gryllotalpa, duration of life of, 39.
  Gryllus campestris, duration of life of, 39.

  Häckel, on reproduction, 72;
    on Perigenesis of the Plastidule, 165;
    on amphigonic reproduction, 272.
  Hare, length of life of, 14.
  Hartlaub, on origin of germ-cells in Obelia, 208.
  Hawk-moths, length of life of imago, 17.
  Helicidae, length of life of, 55, 56, 57.
  Heliotropism, 399.
  Hemiptera, length of life of, 41.
  Hens, length of life of, 36.
  Hensen, on sexual reproduction, 282, 286;
    on difference between germ-plasm and histogenetic nucleoplasm, 343;
    on heredity, 369.
  Heredity, 29, 71, 378;
    defined, 72;
    dependent on continuity of germ-plasm, 104, 168;
    dependent on coalescence of nuclei, 178.
  Hertwig, O., on fusion of nuclei, 174;
    on the influence of gravity in segmentation, 177, 189;
    on polar bodies, 340, 351;
    on process of fertilization, 355.
  Hesperornis, rudimentary wing of, 88.
  Heterogeny, 325.
  Heterogynis, 44.
  Heteroplastides, 130, 131, 134, 139, 146, 153, 204.
  Hildebrandt, on duration of life in plants, 32, 65;
    on cross-fertilization, 309.
  His, on heredity, 166, 390, 412;
    on the transmission of mutilations, 423.
  Hoek, on polar bodies in Balanus, 218.
  Hoffman, on transmission of acquired characters, 407.
  Homoplastides, 122, 139, 146, 202.
  Horse, length of life of, 6, 7;
    in the Falkland Islands, 99.
  Humboldt’s Atur Parrot, 12.
  Hunter, John, experiments in Anabiosis, 25.
  Hyalineae, length of life of, 56.
  Hybrids, 330.
  Hydroids, origin of germ-cells of, 199, 206, 207, 211.
  Hyla, 301, 394.
  Hymenoptera, length of life of, 49.
  Hypermetropia, 89.

  Ichneumons, length of larval life of, 15;
    length of life of imago of, 49.
  Ichthyophthirius multifiliis, fission of, 148, 149.
  Idioplasm, 174, 184, 192, 341;
    not identical with nucleoplasm, 180;
    relation to chromatin, 217;
    two kinds of, 245;
    various combinations of, 276.
  Imago, length of life of, 16.
  Immortality, injurious to species, 24;
    of unicellular organisms, 25, 27, 33.
  Infusoria, immortality of, 25, 72;
    regeneration of lost parts in, 27, 185;
    fission of, 64; encystment of, 117.
  Insects, duration of life amongst, 15;
    duration of larval life of, 15;
    normal death of, 22;
    duration of imaginal life of, 38;
    segmentation of egg of, 73;
    deposition of eggs, 93;
    origin of germ-cells of, 202;
    polar bodies of, 218.
  Instinct, 83;
    origin of, 91, 389;
    used but once in a lifetime, 93.
  Isotropism, of the ovum, 176.
  Ivy, climbing shoots of, 393, 399.

  Jäger, on heredity, 172, 206.
  Jordan, on varieties, 269.
  Julin, on spermatogenesis in Ascaris, 375.

  Kallima, mimicry of, 280, 306.
  Kant, on the transmission of mutilations, 423.
  Karyokinesis, 359, 375.
  Keim, _see_ Germ.
  Kirchner, on development of Volvox, 204.
  Kölliker, on nature of spermatozoa, 175;
    on embryonic cells, 196.

  Lagynus, fission of, 148.
  Lamarck, on use and disuse, 83, 84, 303, 387, 391, 421.
  Lamellibranchiata, length of life of, 55.
  Larvae, length of life of, 15.
  Lasius flavus, length of life of, 50;
    L. niger, 51.
  Lepidoptera, length of life of imagos of, 43, 156;
    parthenogenesis among, 226, 352;
    spermatogenesis in, 375.
  Lepisma saccharina, length of life of, 40.
  Leuckart, on relation of absorbing surface to size of animal, 7;
    on development of Bees, 235;
    on the influence of maternal impressions on the offspring, 445.
  Limnadia Hermanni, 152.
  Limnaeus, length of life of, 56.
  Lion, length of life of, 13.
  Lister, on chromatophores of blind Frogs, 301.
  Locusta, length of life of imago of, 39.
  Lotze, on activity in connection with longevity, 7.
  Lucanus cervus, length of life of imago of, 47.
  Lycaena violacea, length of life of imago of, 44.
  Lynceinae, spermatazoa of, 176.

  Macroglossa stellatarum, length of life of female of, 45.
  Magosphaera planula, 75, 120, 122, 126, 147, 152;
    figure of, 123.
  Magpies, length of life of, 36.
  Mammals, duration of life of, 38.
  Manx cats, 427, 430.
  Maternal impressions, supposed influence on offspring, 444.
  May-flies, length of larval life of, 15;
    length of life of imago of, 16;
    habitat of larvae of, 17;
    shortening of life of, 19;
    death of, 120.
  Meldola, 395.
  Melolontha vulgaris, _see_ Cockchafer.
  Mesozoa, 128.
  Metazoa, 27, 28, 111, 145;
    old age of, 157.
  Metschnikoff, on pole-cells, 197.
  Micellae, 190, 194.
  Mimetic forms, 264, 280.
  Mimosa, ‘after effects’ in, 404.
  Minot, on cyclical development, 199;
    on polar bodies, 214, 225, 340, 345, 353.
  Moina, winter eggs of, 118, 240;
    segmentation of, 199;
    polar bodies of, 218.
  Molluscs, length of life of, 55;
    determined by markings on shell, 14;
    enemies of, 58.
  Monoplastides, 115, 122, 125, 146, 159;
    definition of, 120;
    reproduction of, 149.
  Mouse, length of life of, 6; gestation of, 7.
  Müller, F., on heredity of acquired characters, 320, 322.
  Müller, H., on colours of flowers, 259;
    on nectaries, 307.
  Multicellular organisms, division of labour in, 27.
  Musca domestica, length of life of, 43.
  Musca vomitoria, polar bodies of, 353.
  Myopia, 89.

  Nägeli, 167, 171, 175;
    on idioplasm, 174, 182, 190, 201, 340, 414;
    on inherent tendency to vary, 256, 298;
    on Alpine plants, 269;
    on adaptation, 300;
    on medium of heredity, 318, 355.
  Najadae, length of life of, 56.
  Natica heros, length of life of, 56.
  Nautilus, persistence of, 300.
  Nematodes, polar bodies of, 188;
    nuclear division of ovum of, 234, 368.
  Neuroptera, length of life of, 40.
  Neuroterus, length of life of summer generation, 49;
    of winter generation, 50.
  Nigella, production of double flowers of, 408.
  Nightingale, length of life of, 11, 36.
  Nothnagel, on the cause of epilepsy, 314.
  Nuclear plate, 187.
  Nuclei, behaviour during fission, 118, 188;
    connection of heredity with fusion of, 178;
    influence of, 184;
    influence in regeneration, 185;
    nutrition of, 187.
  Nucleoplasm, 179, 185, 191, 227;
    histogenetic, 213;
    ovogenetic, 213, 230, 243;
    spermogenetic, 220, 243.
  Nussbaum, on heredity, 172, 195, 206;
    on regeneration amongst Infusoria, 185, 200.

  Obelia, origin of germ-cells of, 208.
  Obersteiner, on inheritance of epilepsy in guinea pigs, 311, 313.
  Ophiostomum, karyokinesis in ovum of, 368.
  Orgyia, 44.
  Orth, on the transmission of acquired characters, 411.
  Orthonectides, 120, 126;
    figure of, 127;
    degeneracy of, 130, 131, 141, 152.
  Orthoptera, duration of life of, 39.
  Ostracodes, parthenogenesis of, 294, 325;
    polar bodies of, 350.
  Otostoma Carteri, fission of, 148.

  Palingenia, sub-imago stage of, 19, 40.
  Paludinidae, length of life of, 56.
  Pandorina, 202, 248;
    figure of, 203.
  Pangenesis, theory of, 77, 165, 166, 193, 316, 327.
  Panmixia, principle of, 90, 140, 291, 430.
  Papaver, production of double flowers of, 408.
  Paranucleus, 376;
    of the sperm-cell, 221.
  Parrots, length of life of, 36.
  Parthenogenesis, the origin of, 225, 290, 323, 339;
    not ancestral, 228;
    of bees, 235;
    partial, 238;
    explanation of, 243;
    male, 247;
    of Cynips, 273;
    not perpetual, 283, 285.
  Pasimachus, length of life of, 48.
  Pemphigus terebinthi, length of life of, 41.
  Petromyzon, impregnation of, 175, 247;
    polar bodies of, 218.
  Pfeffer, on chemical attraction of oosphere, 247.
  Pfitzner, on nuclear division, 187.
  Pflüger, on heredity, 70, 175, 355;
    on the inheritance of acquired characters, 81, 390, 422;
    on isotropism of the ovum, 176.
  Phanerogams, fertilization of, 178, 247;
    development of pollen grains of, 222.
  Pheasant, the length of life of the golden, 36.
  Philodina, polar bodies of, 350.
  Phryganea grandis, 41.
  Phylloxera vastatrix, length of life of, 41;
    unequal length of life in two sexes of, 58;
    parthenogenesis of, 294.
  Pieris napi, length of life of, 44.
  Pig, length of life of, 6.
  Pigeon, length of life of, 36;
    cross-breeding of, 332.
  Pike, length of life of, 6.
  Pisidium, length of life of, 56.
  Planorbis, length of life of, 56.
  Plants, duration of life of, 32, 65.
  Polar bodies, 188, 218, 245;
    the significance of, 212, 225, 339;
    of Sponges, 217;
    in parthenogenetic eggs, 249, 345, 383;
    of Rabbit, 339;
    number of, 346;
    significance of second, 353, 362;
    in plants, 377.
  Pole-cells, of Diptera, 197.
  Polistes gallica, ‘workers’ of, 53;
    length of life of males and females, 54.
  Pollen-grains, 222.
  Polyphemus, spermatozoa of, 176;
    summer eggs of, 239;
    polar bodies of, 345.
  Polyplastides, definition of, 120, 122, 125, 159;
    development of, 152.
  Polyzoa, length of life of, 57.
  Poulton, on colours of caterpillars, 394;
    on cats with supernumerary toes, 426.
  Proteus, 87.
  Protomyxa aurantiaca, 149.
  Protozoa, development amongst, 150;
    conjugation of, 287.
  Psorosperms, 150.
  Psychidae, length of life of, 16, 44, 45, 157;
    deposition of eggs of, 18;
    death of female, 63, 132;
    parthenogenesis of, 293.
  Pulex irritans, length of life of, 42.
  Pupa, length of life of, 55.

  Rabbit, polar bodies of, 339.
  Rauber, on heredity, 172.
  Ravens, length of life of, 36.
  Regeneration of lost parts, 65;
    in Infusoria, 185.
  Rejuvenescence, 112, 116, 124, 132, 153, 283.
  Reproduction, original form of, 122;
    effect of monogonic, 273, 275;
    amphigonic, 279, 281, 287.
  Reproductive cells, 27, 28, 111.
  Rhodites rosae, parthenogenesis of, 325.
  Richter, on inheritance of acquired characters, 438.
  Robin, on pole-cells of Diptera, 197.
  Rolph, on conjugation, 286.
  Romanes, on correlation, 389.
  Roth, on heredity, 166, 169.
  Rotifera, unequal length of life in two sexes of, 58;
    polar bodies of, 350.
  Roule, on origin of ova, 222.
  Roux, on the struggle of the parts in the organism, 87, 100;
    on development in altered conditions, 177;
    on forces controlling nuclear division, 231, 361;
    on karyokinesis, 359.
  Rudimentary organs, 88;
    disappearance of, 291;
    not found in parthenogenetic forms, 293.
  Rumia Crataegata, 394.

  Sachs, on reproduction in Mosses, 212;
    on venation, 260, 310;
    on shoots of climbing Ivy, 393, 399.
  Sagitta, segmentation of egg of, 74, 199.
  Saperda carcharias, length of life of imago of, 47.
  Sarcophaga carnaria, length of life of, 42.
  Saturnia pyri, length of life of, 45;
    S. carpini, cocoon of; 94.
  Saturnidae, habits of, 44.
  Saw-flies, ancestors of bees and wasps, 19;
    length of life of imago of, 49, 59.
  Schmidt, on malformations of the ear, 440.
  Schneider, on instincts of perception, 92, 94.
  Schultze, on polar bodies of Amphibia, 340, 352.
  Scytosiphon, male parthenogenesis in, 247.
  Sea-gulls’ eggs, 38.
  Senility, 20, 21, 32, 157.
  Sexes, unequal length of life in the two, 58.
  Sheep, length of life of, 14.
  Sida, spermatozoa of, 176;
    absorption of ova in, 239.
  Siebold, von, on development of Bees, 235.
  Siphonophora, origin of germ-cells of, 202.
  Sirenia, 261.
  Sirex, length of larval life of, 15.
  Smerinthus tiliae, length of life of imago of, 45;
    ocellatus, 395.
  Solenobia triquetrella, length of life of female of, 45;
    death of parthenogenetic forms of, 64, 293.
  Solidago, time of flowering changed, 415.
  Soma, 122, 125, 130, 140, 144, 154, 155.
  Somatic cells, 27, 28, 75, 111, 145, 158.
  Somatogenic characters, 412.
  Somatoplasm, 104, 180.
  Spathegaster, 49.
  Spencer, Herbert, on relation of absorbing surface to size of animal,
     7;
    on influence of diminished nutrition, 241;
    on correlation, 389.
  Spirogyra, on cell-division in, 216.
  Sponges, polar bodies of, 217.
  Spontaneous generation, 34.
  Sprengel, on colours in flowers, 308.
  Squirrel, length of life of, 14.
  Stahl, on protective structures in plants, 260.
  Strasburger, on fertilization of Phanerogams, 178, 340, 355;
    on cyto-idioplasm, 181;
    on influence of nuclei, 184;
    on identity of daughter nuclei, 187;
    on nuclei of sexual-cells, 200, 215, 246;
    on transmission of germ-plasm, 209;
    on cell-division in Spirogyra, 216;
    on development of pollen-grains, 222;
    on parthenogenesis, 237;
    on direction of growth of pollen-tube, 247;
    on heredity, 354, 369, 369;
    on polar bodies in plants, 377.
  Strepsiptera, length of life of, 41;
    unequal length of life in two sexes, 58, 59.
  Succineae, length of life of, 55.
  Swans, length of life of, 37.

  Tagetes, production of double flowers of, 408.
  Talents, transmission of, 95; nature of, 96.
  Tape-worms, 133, 155.
  Termites, duration of life of, 18, 40.
  Terns’ eggs, 38.
  Thuja, dorso-ventral structure of shoots of, 391, 396.
  Tillina magna, 118, 148.
  Toad, length of life of, 6.
  Transmission of acquired characters, 73, 80, 169, 267, 407, 411;
    want of evidence of the, 81;
    unnecessary for the theory of evolution, 83;
    unproved, 105, 142;
    amongst Protozoa, 278;
    supposed botanical proofs of, 387.
  Trematodes, parasitic in Mollusca, 57, 131, 133.
  Trichodinidae, conjugation of, 287.
  Trichoplax adhaerens, 141.
  Tridacna gigas, length of life of, 56.
  Trinchese, on polar bodies, 189, 224.
  Tropaeolum, two kinds of leaves of, 396.
  Turkey, the length of life of, 36.
  Twins, 380.

  Unicellular organisms, immortality of, 25, 27.
  Unio, length of life of, 56, 57.

  Valaoritis, on origin of germ-cells, 195;
    on physiological value of, 246.
  Vanessa cardui, length of life of, 43;
    V. prorsa, 44;
    V. urticae, 44;
    V. levana, deposition of eggs of, 94.
  Variations, always present, 101.
  Ventral canal cell, 223.
  Vertebrata, late origin of reproductive cells in, 74.
  Vespa, 53.
  Viola calcarata, fertilization of, 310;
    tricolor, 414.
  Vitrinae, length of life of, 55.
  Volvocineae, 202, 248.
  Volvox, 204, 248.
  Vorticellidae, conjugation of buds of, 287.
  Vultures, length of life of, 11, 37.

  Wallace, on constancy of number of individuals in successive
     generations, 12;
    on production of death by natural selection, 23.
  Wasps, duration of life of male and female, 18, 53;
    loss of embryonic limbs in development of larva of, 89.
  Westphal, on epilepsy in guinea-pigs, 314.
  Whales, length of life of, 6; adaptation in, 261.
  White mice, experiments in the transmission of mutilations on, 431.
  Will, on origin of ova, 222.
  Wolff, on theory of epigenesis, 316.

  Zacharias, on the inheritance of mutilations, 426.

                                THE END.


                          Transcriber's notes:

Footnotes were collected at the end of each essay and of the appendices
for each essay.

On page 198 the formula (1/_p_) should probably be (1/_pn_).

Obvious typographical errors have been corrected.  Any inconsistencies
in spelling have been left.

“Notes” as used in this volume often means the same as footnotes. The
Appendix to Essay I contains the "Notes" as used in that essay. In the
footnotes to the Appendix to Essay I, one note refers to a note on
another page which has been collected with all the other notes. The
note referred to is indicated by a local note in braces.

In the footnotes for Essay VII this
      “See ‘Nature,’ vol. 36, pp. 491-407.—E. B. P.”
was changed to
      “See ‘Nature,’ vol. 36, pp. 491-507.—E. B. P.”
on supposition of a typo.